# CSS Grid Template Layout Module

## [LONGSTATUS] [DATE: 3 August 2004]

This version:
  https://www.w3.org/TR/[YEAR]/ED-[SHORTNAME]-[CDATE]/
Latest version:
  https://www.w3.org/TR/css3-layout/
Previous version:
  https://www.w3.org/TR/2011/WD-css3-layout-20111129/
Editors:
  Bert Bos (W3C) bert@w3.org
  César Acebal (University of Oviedo)

## Abstract

Above: a typical Web page. Below: the underlying grid template.

CSS is a simple, declarative language for creating style sheets that specify the rendering of HTML and other structured documents. This module defines a typographic grid for CSS. It has features to set up a grid template, to flow content into it, and to absolutely position elements on a grid.

The 'grid' property sets up a matrix of rows and columns and designates empty areas and areas for content (called “slots”). The 'flow' property specifies into which slot an element flows. A grid templates is like a table in that the various slots are aligned in rows and columns and can automatically adapt to each others size, and it is like absolute positioning in that elements can be put in arbitrary slots so that the visual order becomes independent of the document order.

The '::slot()' pseudo-element allows to style the slots (backgrounds, borders, etc.)

The 'grid-position' property can be used instead of the 'top', 'left', 'right' and 'bottom' properties from level 2 to position absolutely positioned elements relative to a grid template, so that they align with the content in the grid.

Grid templates can also be associated with pages in paged media, to create page templates.

Slots can also form “chains” to create non-rectangular regions. A chain of slots is like a series of pages or columns: content is distributed over the slots by breaking it at possible break points (see the 'break-*' properties from [[CSS3-BREAK]]).

The '::fragment()' pseudo-element allows to select (parts of) elements based on whether they fall in a certain slot or not, and thus style content differently in different slots (region-based styling).

## Status of this document

This draft combines ideas from earlier drafts on Template Layout, Grid Positioning [[CSS3GRID]] and Grid Layout [[CSS3-GRID-LAYOUT]].

The section on “CR exit criteria” lists some conditions for this specification to become a W3C Recommendation.

Issues on this draft are mentioned in the text itself (like this) and/or in the on-line Tracker under “Details on Product CSS3 Template Layout.”

## Table of contents

## Introduction

(This section is not normative.)

### Summary and examples

Above: a typical Web page. Below: the underlying grid template. It has four slots, a, b, c and d and the grid template is defined by 'grid: "aaaaaaa" "bccccdd"'.

The styling of a Web page, a form or a graphical user interface can roughly be divided in two parts: (1) defining the overall “grid” of the page or window and (2) specifying the fonts, indents, colors, etc., of the text and other objects. The two are not completely separate, of course, because indenting or coloring a text influences the perceived grid as well. Nevertheless, when one separates the parts of a style that should change when the window gets bigger from the parts that stay the same, one often finds that the grid changes (room for a sidebar, extra navigation bar, big margins, larger images…), while fonts, colors, indents, numbering styles, and many other things don't have to change, until the size of the window becomes extreme.

The properties in this specification work by associating a layout grid with an element. Rather than letting an element lay out its descendants in a single flow, the policy defined in this module gives an element a grid template, which is a set of slots aligned to an invisible grid, where each slot is a separate flow. The descendants are given a 'flow' property to designate the slot into which they flow.

Because layouts on the Web have to adapt to different window and paper sizes, the rows and columns of the grid can be made fixed or flexible in size.

The typical use cases for these properties include:

- Complex Web pages, with multiple navigation bars in fixed positions, areas for advertisements, etc.

- Complex forms, where the alignment of labels and form fields may be easier with the properties of this module than with the properties for tables and margins.

- GUIs, where buttons, toolbars, labels, icons, etc., are aligned in complex ways and have to stay aligned (and not wrap, for example) when the window is resized.

- Paged displays (e.g., printed media) where each page is divided in fixed areas for different kinds of content.

- Complex pages with multiple regions, where the style of the text does not only depend on the type of element it comes from, but also on the region it is displayed in.

Once a grid template has been defined, it can also be used as an alternative coordinate system for absolutely positioned elements. Such elements do not flow into slots, but, by using 'grid-position' (instead of 'top', 'left' etc.) can be aligned to grid lines and thus to the content in the grid.

But for many applications, template-based positioning makes absolute positioning unnecessary. Like absolute positioning, grid templates are especially useful for aligning elements that don't have simple relationships in the source (parent-child, ancestor-descendant, immediate sibling). But in contrast to absolute positioning, the elements are not positioned with the help of horizontal and vertical coordinates, but by flowing them into a template that is very much like a table: The size and alignment of elements are governed implicitly by the rows and columns of the template, and can automatically adapt to the overall size of the element and to the content in each row and column.

In this example, the four children of an element are assigned to four slots (called a, b, c and d) in a 2×2 template. (All mark-up examples in this specification are HTML fragments, unless otherwise stated.)

Each element occupies one slot. In this template, all slots have the same size.

    <style type="text/css">
      dl { grid: "ab"
                 "cd" }
      #sym1 { flow: a }
      #lab1 { flow: b }
      #sym2 { flow: c }
      #lab2 { flow: d }
    </style>
    <dl>
      <dt id=sym1>A
      <dd id=lab1>A is een aapje
      <dt id=sym2>B
      <dd id=lab2>B is de bakker
    </dl>

Templates can also help with device-independence. This example uses Media Queries [[MEDIAQ]] to change the overall layout of a page from 3-column layout for a wide screen to a 1-column layout for a narrow screen. It assumes the page has been marked-up with logical sections with IDs.

    @media all
    {
      body { grid: "aaa"
                   "bcd" }
      #head { flow: a }
      #nav { flow: b }
      #adv { flow: c }
      #body { flow: d }
    }
    @media all and (max-width: 500px)
    {
      body { grid: "a"
                   "b"
                   "c" }
      #head { flow: a }
      #nav { flow: b }
      #adv { display: none }
      #body { flow: c }
    }

The slots in a grid do not overlap, unless with negative margins. Here is how the “zunflower” design of the CSS Zen Garden could be done:

    #container {grid: "abc"}
    #container::slot(a) {box-shadow: 0.5em 0.5em 0.5em; z-index: 1}
    #container::slot(b) {box-shadow: 0.5em 0.5em 0.5em; margin-left: -2em}
    #intro {flow: a}
    #supportingText {flow: b}
    #linkList {flow: c}

Template-based positioning borrows some concepts from table layout, in particular the idea of aligning elements in rows and columns, so that they constrain one another's size. But there are also differences. This example shows some of them. Assume this document fragment:

    <div class=group>
     <div>aa aa aa aa aa aa</div>
     <div>bbb</div>
     <div class=menu>ccccc</div>
    </div>

We can lay it out as three columns, as the following illustrations show. The style sheet would contain the following.

    .group {display: table}
    .group > div {display: table-cell}

Example of rendering with a table.

We can also use a template, in which case the style sheet would contain this:

    .group {grid: "abc"}
    .group > div {flow: a}
    .group > div + div {flow: b}
    .group > div + div + div {flow: c}

By default, the table is as wide as needed to fit its contents. To make sure it is as wide as its containing block, we need to add

    .group {display: table; width: 100%}

That is not needed for the grid template, but, on the other hand, if we want the template to fit its contents, we would need to say so:

    .group {grid: "abc"; width: auto}

(See [[!CSS3BOX]] for the definition of the 'width' property.) The columns of the template are by default all the same size. The columns of the table satisfy certain constraints, but the exact size is not defined. We can make them all the same by adding a rule (see [[!CSS3TBL]]):

    .group {display: table; width: 100%; table-layout: fixed}

Example of rendering with equal columns.

In both styles, we can set a column to a certain size:

    div.menu {width: 3em}

resp.,

    .group {grid: "abc" * * 3em}

Example of rendering with a fixed third column and the other two columns of equal width.

If there is an unknown number of columns (children of the div.group element), the style sheet for the table model will automatically take them into account. The style sheet for the template model, however, creates a template of exactly three columns and can't handle tables with an unknown number of columns. The extra elements will be added into the default slot (in this case the ''a'' slot).

(However, extra columns or rows will be created if necessary to absolutely position elements. See the 'grid-position' property.)

In both models, elements can have borders, but only in the table model can borders be collapsed, which makes setting borders easier in the table model:

    .group {display: table; border-collapse: collapse}
    .group > div {border: solid}

resp.,

    .group > div {border: solid; border-left: none}
    .group > div:first-child {border-left: solid}

In the template model, the order of the elements is explicit, and thus it is possible to reverse the order of the columns:

    .group > div {flow: c}
    .group > div + div {flow: b}
    .group > div + div + div {flow: a}

Example of rendering with the contents of the three columns reversed: the third element is shown in the first slot and the first element in the third slot.

In the table model, the order of the rows and columns is given by the document source and thus can't be changed.

This example shows a way to move notes to the end of a section. “Notes” in this example refers to elements in HTML with a class of “note”. A fragment of HTML such as

    <div class=section>
      <p>The balubious coster of the fifth secter<span
        class=note> The sixth secter coster is a difter
        manon.</span> of The Rollow Carpug mentizes a costernica.
      <p>…
    </div>

with this style sheet

    div.section {
        grid: "*"
              "F"}
    .note {
        flow: F;
        content: counter(note) ".\A0" contents;
        counter-increment: note;
        font-size: smaller}
    .note::before {
        content: counter(note);
        vertical-align: super;
        font-size: larger}

results in a rendering similar to this:

Rendering of a text with footnotes.

The “Generated content for paged media” module [[CSS3GCPM]] is expected to define specific features for footnotes that are both easier and more powerful than this example.

This example shows the use of chained regions: text from region 1 continues in region 2, 3 and 4. And it shows how to use pseudo-elements to style text in a particular region: text in region 1 is bigger than in other regions.

Example rendering

We assume a document fragment similar to this:

    <div id=article>
      <h1>Introduction</h1>
      <p><img src="sunset" alt="">
      <p>This is an example…

      <h1>More Details</h1>
      <p>This illustrates…
      <p>Then, the example…
      <p>Finally, this…
    </div>

The style sheet makes the DIV into a template element with five regions, called A, *, b, c and d. The regions are grouped into two chains: region A on its own, and the chain consisting of *, b, c and d.

    #article {
      grid: "A A c"
            "A A c"
            "* * c"
            "a b c";
      chains: * a b c }

The '*' is a special name. It indicates the default slot, the slot where content goes if it doesn't have an explicit 'flow' property. We could have used a normal letter and added an explicit 'flow' property to the children of the DIV, but by using * we don't have to. All children thus go into region * (and continue in a, b and c as needed). The IMG element goes into region A. We assume for simplicity that there is only one IMG element:

    #article img { flow: A }

The text in region * is bolder, larger and a different color than in the other regions. Also, an H1 that falls into this region is rendered differently from other H1s:

    ::slot(*) {
      font-weight: bold;
      color: #0C3D5F;
      font-size: larger }
    h1::slot(*) {
      color: crimson;
      display: run-in }

(For brevity, the example doesn't show the style rules to set the color and background, to make the text justified, add the left border to the second H1, etc.)

As the image shows, the text of the last paragraph isn't complete and presumably continues somewhere else, but that part is outside the scope of this example. In paged media, we could attach the grid template to a page instead of an element and let the text continue on the next page; we could also make slot c into a multi-column element so that the text forms additional columns to the right of current image (see [[CSS3GCPM]] for some ideas), or we could allow a scrollbar on slot c.

### Dependencies on other modules

This CSS3 module depends on the following other CSS3 modules:

- CSS 2.1 [[!CSS21]] – block container and property content.
- CSS syntax module [[!CSS3SYN]] – interactive vs non-interactive media.
- CSS values and Units module [[!CSS3VAL]] – <string> and <length>.
- CSS box module [[!CSS3BOX]] – properties margin, margin-right, margin-left, width, height, float, display and overflow
- CSS text layout [[!CSS3-WRITING-MODES]] – defines block flow direction and the properties direction and writing-mode. The latter defines when an element has a vertical writing mode (text lines are vertical) or a horizontal writing mode (text lines are horizontal). For brevity, we refer to the former as a vertical element and to the latter as a horizontal element.
- CSS positioning [[!CSS3POS]] – properties z-index, position, top, right, bottom, left.
- CSS fragmentation [[CSS3-BREAK]] – properties break-before, break-after and break-inside; the definition of fragmenter.
- CSS paged media [[CSS3PAGE]] – definition of page area (in particular its width & height).
- CSS background and borders [[CSS3BG]] – properties background, background-image, background-size, box-decoration-break and box-shadow?
- CSS tables [[!CSS3TBL]] – property vertical-align. [or Box Module?]
- CSS Multi-column Layout Module [[CSS3COL]] – property columns.
- CSS Fragmentation Module [[!CSS3-BREAK]] – properties break-before, break-after and break-inside.

Add to the above all the properties that are allowed on slots (see “The '::slot()' pseudo-element”).

It has non-normative (informative) references to the following other CSS3 modules:

- Selectors [[SELECT]]

See section 1.4.2 of CSS level 2 [[CSS21]] for the grammar and other notations that this specification uses in property definitions.

### Values

This specification follows the CSS property definition conventions from [[!CSS21]]. Value types not defined in this specification are defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS modules may expand the definitions of these value types: for example [[CSS3COLOR]], when combined with this module, expands the definition of the <color> value type as used in this specification.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the inherit keyword as their property value. For readability it has not been repeated explicitly.

### A note about accessibility

(This section is not normative.)

The facilities in this specification allow elements from a document to be displayed in a visual order that is to a large extent independent of the order in the document. That may have both positive and negative effects on accessibility. The positive aspect is that it allows the content in the document to be kept in logical order, so that the document is as functional as possible without the style sheet and on media where the style sheet doesn't apply. A negative aspect is that a document that has a defined tab order (the order in which elements get the focus when the tab-key is pressed) will show on the screen with a tab order unrelated to the visual order. It may be necessary to use the keyboard control features of the CSS Basic User Interface module [[CSS3UI]] to ensure that the tab navigation follows the visual order, or to refrain from positioning semantically related elements in different parts of a template.

The following two requirements from the Web Content Accessibility Guidelines (WCAG) 2.0 [[WCAG20]] are particularly relevant. See that specification for more information.

> 1.3.2 Meaningful Sequence: When the sequence in which content is presented affects its meaning, a correct reading sequence can be programmatically determined. (Level A)

> 2.4.3 Focus Order: If a Web page can be navigated sequentially and the navigation sequences affect meaning or operation, focusable components receive focus in an order that preserves meaning and operability. (Level A)

## Declaring templates

Grid templates are declared with the 'grid-template', 'grid-rows', 'grid-columns' and 'grid' properties, defined below.

An element that has a grid template is called a grid element. More precisely: an element is a grid element if (1) it generates a block container box [[!CSS21]] and (2) at least one of 'grid-template', 'grid-rows' or 'grid-columns' has a value other than the initial value.

Block containers are, informally, elements that can have block elements as children, such as blocks, inline blocks or table cells, but not inline elements or replaced elements.

An element's grid ancestor is the nearest ancestor that is a grid element.

Note that not all elements have a grid ancestor. E.g., the root element has none.

The number of columns of a grid element is the number of <col-width> values in 'grid-columns' or the number of columns in 'grid-template', whichever is larger.

The number of rows is the number of <row-height> values in 'grid-rows' or the number of rows in 'grid-template', whichever is larger.

Note that a grid element always has at least one row and one column. Those may, however, have zero size.

### Declaring a template: 'grid-template'

The 'grid-template' property implicitly assigns a grid to an element and defines named slots in the grid. It can also designate a slot as the default slot for content without an explicit flow.

Name: | grid-template
Value: | none | <string>+
Initial: | none
Applies to: | block container elements [[!CSS21]]
Inherited: | no
Animatable: | no
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

Each string consist of one or more asterisks (“*”), letters, periods (“.”), spaces and tabs. Each string represents one row in the template, each symbol other than a space or tab represents one column in that row. Spaces and tabs have no meaning. They may be added for readability.

The symbols in the template have the following meaning

<letter>
  Named slot for content.

  CSS doesn't yet have a type corresponding to single letter. The letters in this module are also used outside of strings (on 'flow', e.g.), so they should be a subset of identifiers. There is a “nmstart” macro in the grammar, but it is not exactly what we want here. E.g., U+0300 (“combining grave accent”) is a valid first character for an identifier, but may not be desirable as a slot in a template. We probably want a definition based on the principle that it is a single letter if it looks like a single letter. This may correspond to the notion of character cluster. (There may also be a connection with the open issue against the CSS syntax about Unicode normalization. We want é to match é, no matter if the author typed U+0301 U+0065 or U+00C9.)

*
  (Asterisk.) Default slot for content (explained below).
.
  (Period.) Blank space.

Multiple identical letters in adjacent rows or columns form a single slot that spans those rows and columns. Ditto for multiple “*”s. Slot names are case-sensitive, so uppercase and lowercase letters are considered to be different letters.

Non-rectangular slots and multiple slots with the same letter are illegal. A template without any letter or “*” is illegal. A template with more than one “*” slot is illegal. These errors cause the declaration to be ignored.

Note: non-rectangular and disconnected regions may be permitted in a future update of CSS.

Rows with fewer columns than other rows are implicitly padded with periods (“.”).

Each slot (letter or “*”) acts as a block element for its contents.

If the value is ''none'', then no explicit slots are defined. If the element is a grid element, then the element instead has an implicit template consisting of a single '*' slot.

Note that an element is a grid element in this case if 'grid-columns' and 'grid-rows' are not both ''auto''.

For example, the following two sets of style rules are equivalent:

    DIV {
      grid-template: none;
      grid-rows: 10em 10em 10em;
      grid=columns: * * * * }

and

    DIV {
      grid-template: "****" "****" "****";
      grid-rows: 10em 10em 10em;
      grid=columns: * * * * }

### Specifying the widths of columns: 'grid-columns'

Name: | grid-columns
Value: | auto | <col-width>+
Initial: | auto
Applies to: | block container elements
Inherited: | no
Animatable: | yes, between grids with the same number of tracks
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

The 'grid-columns' property specifies the sizes of the columns of a grid.

Where

    <col-width> = <length> | <percentage> | * | <fraction> |
      <minmax> | min-content | max-content | auto
    <minmax> = minmax( <col-width> , <col-width> )

The definition of minmax isn't correct, it should only accepts certain values.

Each <col-width> sets the width of a column, the first value for the first column, the second for the second columns, etc.

If there are fewer <col-width> values than the number of columns in the element, or if the value is ''auto'', the missing columns widths are all '*'.

Each <col-width> can be one of the following:

<length>
  An explicit width for that column. Negative values are illegal.
<fraction>
  (A non-negative number followed by ''fr''.) A fraction of the remaining space, see Fraction values below.
*
  (Asterisk.) All columns with a width of '*' have the same width.
<percentage>
  Expresses a size as a percentage of the element's a-priori content width. When the width of the grid element is dependent on content, the result is undefined.
max-content
min-content
  The column's width is determined by its contents. See the algorithm below.
minmax(p,q)
  The column's width is constrained to be greater than or equal to p and less than or equal to q. p and q stand for [ <length> | max-content | min-content | * ]. There may be white space around the p and q. If q < p, then q is ignored and ''minmax(p,q)'' is treated as 'minmax(p,p)'.
auto
  Equivalent to 'minmax(min-content, max-content)'.

### Specifying the height of rows: 'grid-rows'

Name: | grid-rows
Value: | auto | <row-height>+
Initial: | auto
Applies to: | block container elements
Inherited: | no
Animatable: | yes, between grids with the same number of tracks
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

The 'grid-rows' property specifies the heights of the rows of a grid.

    <row-height> = <length> | <percentage> | * | <fraction> |
      <minmax> | min-content | max-content | auto
    <minmax> = minmax( <row-height> , <row-height> )

The definition of minmax isn't correct, it should only accepts certain values.

Note that the syntax is the same syntax as for 'grid-columns'

If there are fewer <row-height> values than the number of rows in the element, or if the value is ''auto'', the missing row heights are all ''auto''.

Each <row-height> can be one of the following:

<length>
  An explicit height for that row. Negative values are illegal.
<fraction>
  (A non-negative number followed by ''fr''.) A fraction of the remaining space, see Fraction values below.
* (asterisk)
  All rows with a height of '*' have the same height.
<percentage>
  Expresses a size as a percentage of the element's a-priori content height.

  When the height of the grid element is dependent on content, the result is undefined.

max-content
min-content
  The row's height is determined by its contents. See the algorithm below.
minmax(p,q)
  The row's height is constrained to be greater than or equal to p and less than or equal to q. p and q stand for [ <length> | max-content | min-content | * ]. There may be white space around the p and q. If q < p, then q is ignored and ''minmax(p,q)'' is treated as 'minmax(p,p)'.
auto
  Equivalent to 'minmax(min-content, max-content)'.

### The 'grid' shorthand property

Name: | grid
Value: | none | [ [ <string> [ / <row-height> ]? ]+ ] <col-width>*
Initial: | none
Applies to: | block container elements
Inherited: | no
Animatable: | see individual properties
Percentages: | see individual properties
Computed value: | see individual properties
Canonical order: | N/A

The 'grid' property is a shorthand for 'grid-template', 'grid-columns' and 'grid-rows' and is equivalent to setting those properties as follows:

- 'grid-template' is set to the list of <string> values.
- 'grid-rows' is set to the list of <row-height> values, with any omitted ones set to ''auto''.
- 'grid-columns' is set to the list of <col-width> values, or to ''auto'' if there are none.

For example, the rule

    grid: "abc" "abd"/4em * 10em

is the same as

    grid-template: "abc" "abd";
    grid-rows: auto 4em;
    grid-columns: * 10em;

Note the ''auto'' keyword that was implied in the shorthand, but must be made explicit in the 'grid-rows' property.

Does it look better with the column sizes first instead of last? There would be no need for a slash:

> none | <col-width>* [ <string> <row-height>? ]+

E.g.:

    grid: 5em 1em  *  1em 10em
          "a   .   b   .   c"  2em
          ".   .   .   .   ."  1em
          "d   .   e   .   f"
          ".   .   .   .   ."  1em
          "g   .   h   .   i"  2em

### Default slots

Every grid element has a default slot. If there is an asterisk (“*”) in the template, then that slot is the default. If there is no asterisk, then the first letter in the template defines the default slot. If there is no letter either, then…

… define where a grid element's content goes if it doesn't have any slots at all.

For example, if the template is defined by 'grid-template: "..." "..c" "abb" "abb"', then “c” is the default slot.

If the grid has an implicit template (i.e., 'grid-template' is ''none''), then its single slot is the default slot.

All content of a grid element that is not inside another flow (i.e., not inside a float, not absolutely positioned, etc.) is flowed into the default slot. In particular, any text content of the grid element itself is in that default flow.

For example, in this document fragment

    <DIV STYLE="grid: 'ab*'">
     <IMG STYLE="flow: a" SRC="image.png" ALT="Foo">
     This is an
     <EM STYLE="flow: *">emphasized</EM>
     <EM STYLE="flow: b">(note well!)</EM>
     sentence.
     <
    </DIV>

The three slots of the grid contain:

Slot a
  The image
Slot b
  The text “(note well!)”
Default slot
  The text “This is an emphasized sentence.”

(The 'flow' property is defined below.)

### Calculating the size of the grid

For the purpose of the calculations below, each slot (letter or “*”) in a grid has four dimensions associated with it, called MINW (“minimum width”), PREFW (“preferred width”), MINH (“minimum height”) and PREFH (“preferred height”). We can think of them as the minimum and preferred dimensions of the slot in isolation, i.e., if it wasn't constrained by other slots in the same row or column. They are defined as follows:

MINW
  - If the slot spans only columns whose width does not depend on the contents of the column (i.e., the column widths are specified with only <length> values, '*', <fraction> values, or any combination of those), then MINW is 0. (Note that <length> values can be specified with 'calc()' as well, see [CSS3VALUES]].)
  - Otherwise, if the slot is vertical, then MINW is the block dimension of the slot, plus its horizontal padding, border and margin (i.e., any 'margin-left', 'margin-right', etc., that is set on it by means of the '::slot()' pseudo-element). Any ''auto'' margins are counted as 0 in this case.
  - Otherwise, if the slot spans at least one column whose width is specified as ''max-content'' (or 'minmax()' with a first argument of ''max-content''), then MINW is the intrinsic preferred width (see [[!CSS3BOX]]) of the slot plus its horizontal margin, border and padding. Any ''auto'' margins are counted as 0 in this case.
  - Otherwise (i.e., the slot spans at least one column whose width is specified as ''min-content'' or ''minmax()'' with a first argument of ''min-content'', but no ''max-content'') MINW is the intrinsic minimum width (see [[!CSS3BOX]] of the slot, plus its horizontal padding, border and margin.
PREFW
  - If the slot spans only columns whose width does not depend on the contents of the column (i.e., the column widths are specified with only <length> values, '*', <fraction> values, or any combination of those), then PREFW is infinite.
  - Otherwise, if the slot is vertical, then PREFW is the block dimension of the slot, plus its horizontal padding, border and margin.
  - Otherwise, if the slot spans at least one column whose width is specified as ''max-content'' (or 'minmax()' with a first argument of ''max-content''), then PREFW is the intrinsic preferred width (see [[!CSS3BOX]]) of the slot plus its horizontal margin, border and padding.
  - Otherwise PREFW is the intrinsic minimum width (see [[!CSS3BOX]] of the slot, plus its horizontal padding, border and margin.
MINH
  - If the slot spans only rows whose height does not depend on the contents of the column (i.e., the row heights are specified with only <length> values, '*', <fraction> values, or any combination of those), then MINH is 0.
  - Otherwise, if the slot is horizontal, then MINH is the block dimension of the slot, plus its vertical padding, border and margin
  - Otherwise, if the slot spans at least one row whose height is specified as ''max-content'' (or 'minmax()' with a first argument of ''max-content''), then MINH is the intrinsic preferred height (see [[!CSS3BOX]]) of the slot plus its vertical margin, border and padding.
  - Otherwise MINH is the intrinsic minimum height (see [[!CSS3BOX]] of the slot, plus its vertical padding, border and margin.
PREFH
  - If the slot spans only rows whose height does not depend on the contents of the column (i.e., the row heights are specified with only <length> values, '*', <fraction> values, or any combination of those), then PREFH is infinite.
  - Otherwise, if the slot is horizontal, then PREFH is the block dimension of the slot, plus its vertical padding, border and margin.
  - Otherwise, if the slot spans at least one row whose height is specified as ''max-content'' (or 'minmax()' with a first argument of ''max-content''), then PREFH is the intrinsic preferred height (see [[!CSS3BOX]]) of the slot plus its vertical margin, border and padding.
  - Otherwise PREFH is the intrinsic minimum height (see [[!CSS3BOX]] of the slot, plus its vertical padding, border and margin.

For example, the MINW values of the slots in this grid

    grid: "  a  a  .  "
          "  b  .  c  "
           auto * 10em

are as follows:

a
  ''min-content'' (because the slot spans at least one column with a width specified as ''min-content'' or ''auto'').
b
  ''min-content'' (ditto).
c
  0 (because the slot spans only columns with a width that is independent of the content).

The UA must choose the widths and heights of all columns and rows such that the following constraints are satisfied.

1. If the element has an a-priori known content height, then the sum of the heights of all rows must be equal to the element's height.

   For example, the two rows in this grid must be 8em each so that the total height matches the height of the element:

       div {
         height: 16em;
         grid: "a.b"/*
               "ac."/* }

2. If the grid element has an a-priori known content width, then the sum of the widths of all columns must be equal to the element's width.

   For example, the three columns in this grid must be 20em each:

       div {width: 60em; grid: "abc"}

3. Each row with a height specified as a <length> must have exactly that height.
4. Each column with a width specified as a <length> must have exactly that width.
5. All rows with a height specified as '*' must have the same height.

   If we have both '*' and ''fr'', then '*' will be defined as '1fr' and we can drop this rule and the next. Otherwise either these two rules or the next two must be removed.

6. All columns with a width specified as '*' must have the same width.
7. For any pair (i,j) of rows whose <row-height> values hi and hj are both specified in ''fr'', the computed heights Hi and Hj must be such that Hi * hj = Hj * hi. (I.e., their heights are proportional to their number of ''fr'' units.)
8. For any pair (i,j) of columns whose <col-width> values wi and wj are both specified in ''fr'', the computed width Wi and Wj must be such that Wi * wj = Wj * Wi. (I.e., their widths are proportional to their number of ''fr'' units.)

9. Each row that contains slots that span only one row and no slots that span more than one row, must not be higher than the largest PREFH of all slots in the row plus the amount needed to vertically align the horizontal slots that have a 'vertical-align' of ''baseline''.

   For example, the second row in this grid

       grid: "a.c.." / 5em
             "....b" / auto

   must not be taller than the height (block dimension) of slot b. The first row contains a slot that spans two rows (slot a), so this rule does not apply to that row.

10. Each column that contains slots that span only one column and no slots that span more than one column, must not be wider than the largest PREFW of all slots in the column plus the amount needed to align the vertical slots that have a 'vertical-align' of ''baseline''.

    For example, in this grid

        grid: "ac"
              "ab"
              auto *

    the first column must not be wider than the PREFW of slot a. Both slots in the second column have an infinite PREFW, so this rule effectively puts no constraint on that column.

11. Each slot must be at least as wide as its MINW.
12. Each slot must be at least as high as its MINH.

If it is impossible to choose such widths and heights, then try without constraint 1. If it is still impossible, try without constraint 2 instead. And, finally, try with both 1 and 2 dropped.

For example, the sum of the row heights in this example can never be the same as the height of the element:

    div {
      height: 20em;
      grid: "a b c" / 7em
            "a . c" / 7em }

The first constraint is therefore ignored, the rows are 7em each and 6em of space below the grid remains empty.

If there are multiple solutions, and constraint 1 was dropped or did not apply, then the sizes must additionally be chosen such that the sum of the heights of the rows is minimized.

If there are still multiple solutions, and constraint 2 was dropped or did not apply, then the sizes must additionally be chosen such that the sum of the widths of the columns is minimized.

The calculation is intended to be symmetric in width and height (except for the case when both width and height are fixed and the grid is over-constrained, when a solution that ignores the height is tried before trying one that ignores the width).

For example, there would be multiple solutions for this grid:

    <DIV STYLE="float: left; grid: 'a.b'">
     <P STYLE="flow: a">Two words
     <P STYLE="flow: b">Only three words
    </DIV>

The columns must have equal width, but there is no other constraint on the width. They could be narrow:

> `Two
> words
> ` `
> ` `Only
> three
> words`

or wider:

> `Two words` `` `Only three words`

or even wider still, e.g.:

> `Two words` `` `Only three words`

The rule to minimize height excludes the first, narrow solution. The rule to minimize width excludes the third and all wider solutions. So the second layout, the narrowest that has all words on one line, is the correct one.

(This example assumes the width of the floating DIV's containing block is large enough. The default width of a float is actually ''fit-content'', and thus if the containing block is too narrow, the result will be narrower, too, because the calculation will have to be redone using that width as the a-priori width for the DIV.)

The width isn't known a-priori, if, e.g., 'width' is ''auto'' and the element is floating, absolutely positioned, inline-block or a child of a block with vertical writing mode.

An extra step may be necessary in paged media if a page break occurs inside a template (only in the case of an element-based template, see below). If the template, after computing the width and height, is too big to fit on the current page, and if a suitable break point exists, the part of the template after that break point is put on the next page. The width of the containing block on that page may be different if that page has different margins from the current page (see [[!CSS3PAGE]]) and thus the width and height of that part of the template must be recalculated in the new context.

Note that the widths of the columns can be completely determined before laying out any of the contents as long as there are no columns with a <col-width> of ''min-content'' or ''max-content''.

Do we define restrictions or approximations for slots that are part of a chain to avoid complicated optimization algorithms?

Note: In a future update of CSS, rows might get a property to specify how the height of that row is adjusted in case the above calculation yields a template that is less tall than the element itself.

The height of a slot is measured as if the slot had one anonymous block as a child that contains all the slot's contents and the anonymous block is a flow root (see [[!CSS3BOX]]).

This example divides the window in three rows and three columns, separated by 1em of white space. The middle row and the middle column are flexible, the others are fixed at a specific size. The first column is 5em wide, the last one 10em.

    <style type="text/css">
      body {
        height: 100%;
        grid: "a   .   b   .   c"  /2em
              ".   .   .   .   ."  /1em
              "d   .   e   .   f"
              ".   .   .   .   ."  /1em
              "g   .   h   .   i"  /2em
              5em 1em  *  1em 10em}
      #logo {flow: a}
      #motto {flow: b}
      #date {flow: c}
      #main {flow: e}
      #adv {flow: f}
      #copy {flow: g}
      #about {flow: h}
    </style>
    <p id=logo><img src=...
    <p id=motto>Making Web pages since 1862
    <p id=date>August 2, 2004
    ...

[Add example with three columns, first two as narrow as possible, third one taking up all remaining space.]

## Putting content into a grid element

This section until the next subsection is not normative.

Grids can be used in two different ways to position content. The first way is to flow content into slots (either into explicit slots identified by letters, or the default slot “*”). Each slot is an individual flow root (“establishes a block formatting context” in the terminology of CSS level 2), very much like a table cell.

The second way is to absolutely position elements using four grid lines to specify the positions of the four margin edges of the element. Like other positioned elements [[!CSS21]], these elements can overlap each other and the 'z-index' property applies to them.

### Flowing content into slots: 'flow'

The 'flow' property adds an element to a slot.

Name: | flow
Value: | auto | <identifier> | '*' | same
Initial: | auto
Applies to: | elements with 'position' equal to ''static'' or ''relative''
Inherited: | no
Animatable: | no
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

If the value is not ''auto'', the element is added to the flow of the given slot, instead of to the flow of its parent.

If the element has no grid ancestor, or that grid ancestor has no slot of the given name, the property is treated as if it were ''auto''.

A value of ''same'' means the element is put into the same slot as the nearest preceding element in document order that (1) has the same grid ancestor, (2) to which the 'flow' property applies and (3) whose 'flow' property is not ''auto''.

If the letter refers to a slot that doesn't exist in the element's grid ancestor (or there is no grid ancestor) there are several possibilities:

1. The element is not positioned (i.e., it is positioned in the current flow).
2. The letter is looked for in the grid ancestor's own grid ancestor, etc., recursively. If no slot of that name is found in any of them, then the element is not positioned.
3. The element is positioned in the default slot (i.e., as if 'flow: *' had been specified); or not positioned at all, in case there is no grid ancestor.
4. The letter is looked for in the grid ancestor's own grid ancestor, etc., recursively. If no slot of that name is found in any of them, then the element is positioned in the default slot.

All content flowed into the same slot, whether explicitly with 'flow' or implicitly by flowing into the default slot, forms a single flow, with content in document order. The slot establishes a block formatting context and becomes the containing block of the resulting content flow. The boxes of elements flowed into the same slot explicitly (by means of 'flow') are each others siblings in the slot.

The content flowed into a slot does not inherit properties from the slot.

For example, the style sheet

    BODY {grid: "a." ".b"}
    .paris {flow: a}
    .london {flow: b}

with this document

    <DIV CLASS=london>
     <P>The...
     <DIV CLASS=paris>
      <P>The...
      <DIV CLASS=london>
       <P>The...
      </DIV>
     </DIV>
    </DIV>

causes the second and third DIVs to be taken out of their parents. The second DIV becomes the first child of slot a (i.e., of the pseudo-element called '::slot(a)'). The third DIV becomes the sibling of the first DIV, because both are added to slot b independently.

Note that 'flow' applies to floating elements: they are floated relative to their containing block, and if their 'flow' property indicates a slot in a grid, that slot is their containing block. See also “Floating elements inside templates” below.

A common markup in HTML for illustrations with captions is as follows:

    <div class=figure>
     <p><img src="paul.jpg" alt="...">
     <p class=caption>A pond in a playground in Amsterdam
    </div>

The caption can be put above the image by using a template as follows:

    div.figure {grid: "aaa"
                      ".b."
                      * min-content *}
    div.figure p {flow: b}
    div.figure p.caption {flow: a; text-align: center}

The caption can be wider than the image and the image will be centered.

When the figure is floating, it is probably better to not let the caption become wider than the image (unless the caption cannot be made narrow enough):

    div.figure {float: right; grid: "a" "b" min-content}
    div.figure p {flow: b}
    div.figure p.caption {flow: a; text-align: center}

In this example, a form is laid out on a grid, with two labels and two input boxes and a submit and a reset button:

    form {
      border: thin solid;
      grid: "aaaa.bbbb"
            "........."/1em
            "cccc.dddd"
            "........."/1em
            "...ee..ff" }
    label[for=minv] { flow: a }
    input#minv { flow: b; display: block }
    label[for=maxv] { flow: c }
    input#maxv { flow: d; display: block }
    input[type=submit] { flow: e; display: block }
    input[type=reset] { flow: f; display: block }

Here is the fragment of HTML that the style is applied to:

    <form action="./">
      <label for=minv>Enter minimum value:</label>
      <input id=minv name=minv>
      <label for=maxv>Enter maximum value:</label>
      <input id=maxv name=maxv>
      <input type=submit value="OK">
      <input type=reset value="Reset">
    </form>

The addition of 'display: block' causes the form controls to use the width computation of blocks, in other words: they will be as wide as their containing block, which in this case means that they will be as wide as the slot they are assigned to. Without it, they would be inline elements and just be left-aligned in their slots.

Possible rendering of the example.

This example shows that templates can be nested. The body has two columns. The #content element that goes into the second column has itself another template, into which the various “modules” are placed.

Possible rendering of the nested templates. (The borders are added for clarity, they don't occur in the style rules below. The red border is the inner template.)

For clarity, the inner template uses different letters for the slots than the outer template. This is not required.

    <style type="text/css">
    body {
         grid: "a   b"
               10em *;
    }
    #nav {
         flow: a;
    }
    #content {
         flow: b;
         grid: "c   .   d   .   e   "
               ".   .   .   .   .   "/1em
               ".   .   f   .   .   "
                *  1em  *  1em  *;
    }
    .module.news {
         flow: c;
    }
    .module.sports {
         flow: d;
    }
    .module.personal {
         flow: e;
    }
    #foot {
         flow: f;
    }
    </style>
    <body>
    <ul id="nav">
         <li>navigation</li>
    </ul>
    <div id="content">
        <div class="module news">
             <h3>Weather</h3>
             <p>There will be weather</p>
        </div>
        <div class="module sports">
             <h3>Football</h3>
             <p>People like football.</p>
        </div>
        <div class="module sports">
             <h3>Chess</h3>
             <p>There was a brawl at the chess tournament</p>
        </div>
        <div class="module personal">
             <h3>Your Horoscope</h3>
             <p>You're going to die (eventually).</p>
        </div>
        <p id="foot">Copyright some folks</p>
    </div>

    </body>

This example shows the use of ''same'' to put `DD` elements in the same slot as the preceding `DT`.

    ...
    DL {grid: "a.b.c"  * 2em * 2em *}
    DT.mineral {flow: a}
    DT.animal {flow: b}
    DT.plant {flow: c}
    DD {flow: same; margin-left: 1em}
    ...
    <DL>
     <DT class=animal>falcon
     <DD>This bird of prey...
     <DT class=animal>rabbit
     <DD>Local restaurants...
     <DT class=mineral>granite
     <DD>This rock occurs...
     <DT class=plant>olive
     <DD>The fruit of...
     <DT class=mineral>limestone
     <DD>A rock composed of...
     <DT class=plant>pine
     <DD>The western half...
    </DL>

Possible rendering of the `DL` list, with items sorted into three columns.

An alternative to ''same'' may be to create selectors with regular-expression-like capabilities: 'DT.plant + DD* + DD {flow: c}' selects a DD that follows zero or more DDs that follow DT.plant.

Does a percentage 'height' work on an element that is flowed into a slot? If the grid template assigns a fixed height to the slot, it is obvious what a percentage means, but if the slot's height is ''max-content' and an element in the slot has 'height: 110%', there is a conflict…

### Absolute positioning using a grid: 'grid-position-x' and 'grid-position-y'

Trying to combine 'grid-row/column' and 'grid-row/column-span', because they should not cascade independently. (If one style rule tries to place by start/end lines, and another tries to place by span, the result of the cascade will be a mess.)

The 'grid-position-x' and 'grid-position-y' properties can absolutely position an element relative to a grid.

Name: | grid-position-x, grid-position-y
Value: | auto | [ <identifier> | <grid-line> ] [ + <span> | - [ <identifier> | <grid-line> ] ]?
Initial: | auto
Applies to: | elements with 'position: absolute'
Inherited: | no
Animatable: | no
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

Where

    <grid-line> = <integer> | same | next
    <span> = <integer>

If the value is ''auto'', the element is positioned with 'top' and 'bottom' (for 'grid-position-y') or 'left' and 'right' (for 'grid-position-x', see [[!CSS21]]. The other values mean that the computed values of 'top'/'bottom' or 'left'/'right' are ignored and their used values are set as defined below.

If the element has no grid ancestor, the value is interpreted as if it were ''auto''.

An <identifier> refers to a slot in the grid ancestor.

If an <identifier> refers to a non-existent slot, the whole value is interpreted as if it were ''auto''.

If <grid-line> is an integer, it refers to a grid line, i.e., the horizontal line at the edge of a row or the vertical line at the edge of a column. The top of the first row of a grid template is the horizontal grid line with number 1, the top of the second row is 2, etc. up to N+1 for a grid template with N rows. Similarly, the vertical grid lines are numbered 1 to M+1 for a grid template with M columns.

It is an error if the integer is ≤ 0.

The keyword ''same'' stands for the grid line specified by the nearest preceding element in document order with the same grid ancestor to which 'grid-position-x' or 'grid-position-y' applied. If there is no such element, or if it specified ''auto'', then ''same'' stands for '1'.

The keyword ''next'' stands for next the grid line after the one specified by the nearest preceding element in document order with the same grid ancestor to which 'grid-position-x' or 'grid-position-y' applied. If there is no such element, or if it specified ''auto'', then ''next'' stands for '1'.

A <span> is a non-negative integer. It indicates the number of rows or columns the element spans, as defined below.

It is an error if the span is < 0.

The values are of the following forms:

<identifier>
  Equivalent to '<identifier> - <identifier>' (twice the same identifier).
<grid-line>
  Equivalent to '<grid-line> + 1' (i.e., a position and a span of 1).
<identifier> - <identifier>
  'Top' (or 'left') is set to the top (or left) margin edge of the first slot. 'Bottom' (or 'right') is set to the bottom (or right) margin edge of the second slot.
<identifier> - <grid-line>
  'Top' (or 'left') is set to the top (or left) margin edge of the first slot. 'Bottom' (or 'right') is set to the given grid line.
<grid-line> - <identifier>
  'Top' (or 'left') is set to the indicated grid line. 'Bottom' (or 'right') is set to the bottom (or right) margin edge of the second slot.
<grid-line> - <grid-line>
  'Top' (or 'left') is set to the grid line indicated by the first number. 'Bottom' (or 'right') is set to grid line given by the second number.
<identifier> + <span>
  'Top' (or 'left') is set to the top (or left) margin edge of the first slot. The <span> specifies the number of rows (or columns) spanned, i.e., 'bottom' (or 'right') is set to the grid line <span> rows below (or columns to the right) of that edge.
<grid-line> + <span>
  'Top' (or 'left') is set to the grid line given by the first number The second number specifies the number of rows (or columns) spanned, i.e., 'bottom' (or 'right') is set to the grid line <span> rows below (or columns to the right) of the first.

For example, these rules

    DIV {grid: "ab"
               "cd"}
    P {position: absolute; grid-position-x: d; grid-position-y: a}

mean that any P spans the same rows as slot a and spans the same columns as slot d. In this case, that means it coincides with slot b. All of the following are thus equivalent:

    P {position: absolute; grid-position-x: b; grid-position-y: b}
    P {position: absolute; grid-position-x: b + 1; grid-position-y: b + 1}
    P {position: absolute; grid-position-x: b - b; grid-position-y: a - a}
    P {position: absolute; grid-position-x: 2 + 1; grid-position-y: 1 - 2}
    P {position: absolute; grid-position-x: 2; grid-position-y: 1}

Use negative number to count from the right/bottom?

This needs to be worked into the constraints for the calculation of the grid size, so that these absolutely positioned elements also contribute to the calculation of ''min-content'' and ''max-content''.

When using <grid-line> or <span> (but not when using <identifier>), it is possible to refer to grid lines that do not exist in the grid declared with 'grid-columns'/'grid-rows'. In such a case, the grid is automatically extended with the necessary rows or columns. The sequence of sizes given by 'grid-columns'/'grid-rows' is repeated as often as necessary to create exactly the required number of rows and column.

For example, you can transpose a table such as

    <TABLE>
     <TR><TD>A1 <TD>A2 <TD>A3
     <TR><TD>B1 <TD>B2 <TD>B3
     ...
     <TR><TD>K1 <TD>K2 <TD>K3
     ...
    </TABLE>

to display it somewhat like this

    A1 B1... K1...
    A2 B2... K2...
    A3 B3... K3...

with style rules like this:

    TABLE {grid-columns: auto; grid-rows: auto}
    TD:first-child {grid-position-x: next; grid-position-y: 1}
    TD {grid-position-x: same; grid-position-y: next}

ISSUE-201: Terra Informatica's HTMLayout program proposes an interesting shortcut: if a slot's name is a number (instead of a letter), it refers directly to the child with that number. That child is absolutely positioned on top of that slot and doesn't need 'position', 'grid-position-x' or 'grid-position-y' to be set. See the documentation by Andrew Fedoniouk and Ivan Goroun. E.g., `<ul style="grid: '2 1'"><li>one <li>two </ul>` puts “two” on the left and “one” on the right. (Note that this requires that the names of slots are separated by spaces.) It is thus also a shortcut for some applications of 'flex-order'.

### The 'grid-position' shorthand property

Name: | grid-position
Value: | <grid-position-x> [ / <grid-position-y> ]?
Initial: | auto
Applies to: | elements with 'position: absolute'
Inherited: | no
Animatable: | no
Percentages: | N/A
Computed value: | specified value
Canonical order: | per grammar

This is a shorthand for 'grid-position-x' and 'grid-position-y'. If <grid-position-y> is omitted, it is the same as <grid-position-x>.

For example, these rules

    DIV {grid: "ab"
               "cd"}
    P {position: absolute; grid-position: d / a}

position any P to coincide with slot b. All of the following are thus equivalent:

    P {position: absolute; grid-position: b / b}
    P {position: absolute; grid-position: b + 1 / b + 1}
    P {position: absolute; grid-position: b - b / a - a}
    P {position: absolute; grid-position: 2 + 1 / 1 - 2}
    P {position: absolute; grid-position: 2 / 1}
    P {position: absolute; grid-position: b}

## Styling slots: the ''::slot()'' pseudo-element

The slots of a grid element can be individually addressed with the ''::slot()'' pseudo-element.

For example, the following sets the background and vertical alignment of some of the slots in a template:

    body { grid: "aaa"
                 "bcd" }
    body::slot(b) { background: #FF0 }
    body::slot(c), body::slot(d) { vertical-align: bottom }

Only the following properties apply to the ''slot()'' pseudo-element.

- background properties
- 'vertical-align' (see “vertical alignment” below)
- 'overflow' (see the sections on vertical alignment and paged media below)
- 'writing-mode'
- 'direction'
- 'box-shadow'
- border properties
- padding properties
- margin properties
- 'box-decoration-break'
- column properties

Should 'content' be allowed? or should instead '::slot(a)::before' be possible (and ditto with '::after')? The content would be at the start of the flow, before any elements flowed into the slot and even before the grid element's '':before'' pseudo-element, if the slot is the default slot. If content applies, then most other properties apply, too, to set the style of that content: color, font, text-decoration…

Should the 'content' property allow 'element()' from GCPM? You could then repeat the heading of an article at the top of each slot in case the article is split over several chained slots or several pages. (See also the issue about “continued on.”)

ISSUE-37: Can a slot have a border and if so, where is it drawn?

The background of a slot is drawn immediately on top of the background of the element itself. E.g., the background set on ''P::slot(a)'' is immediately in front of the background set on ''P''.

Margins on slots do not collapse, neither with other slots, nor with elements inside the slot.

Margins aren't really necessary on slots. You can add '.' cells in the grid template to make empty space.

## Styling the contents of slots: '::fragment()'

The ':fragment(a)' pseudo-element selects the part of an element that is inside slot a (similar to how '::first-line' selects the part of an element that is on the first line).

Inherited properties…

Non-inherited properties…

## Rendering of grid elements

Grid elements influence the stacking order, page breaks, the position of floats, etc. of the content inside them.

### Vertical alignment of the contents of slots

The 'vertical-align' property of a '::slot()' pseudo-element can be used to align elements vertically in a slot (or horizontally, if the slot is vertical). The effect is as if the hypothetical anonymous block that contains the slot's contents is positioned as defined below.

For the purpose of this section we define the tail edge and head edge of a box as a writing-mode-dependent edge as follows:

Value of 'writing-mode' | Meaning of “head” | Meaning of “tail”
--- | --- | ---
''horizontal-tb'' | top | bottom
''vertical-rl'' | right | left
''vertical-lr'' | left | right

E.g., if a box is horizontal, “head edge” means the top edge.

bottom
  The content of the slot is aligned to the tail edge of the slot: the tail margin edge of the anonymous block coincides with the tail margin edge of the slot.
middle
  The content of the slot is centered in the slot: the distance between the head margin edge of the anonymous block and the head margin edge of the slot is equal to the distance between the tail margin edge of the anonymous block and the tail margin edge of the slot.

  (Note that if the content overflows the slot, it will overflow at both edges.)

baseline
  The anonymous block that encloses the content is placed as far as possible in the direction against the block flow direction under two constraints:
  1. The head margin edge of the anonymous block may not be beyond the head margin edge of the slot.
  2. If the content has a relevant first line (see below), then the baseline of that line must align with the baselines of the relevant first lines in all other slots with the same 'writing-mode' and the same 'vertical-align' in the same row (if the slot is horizontal) or the same column (if the slot is vertical). A slot has a relevant first line, if the content has a first line (ignoring any lines inside floats) and that first line has the same 'writing-mode' as the slot itself.

  For example, for a horizontal slot, this means that the first baseline must be aligned with the first baselines of all other horizontal slots in the row that also have 'vertical-align: baseline'.

<percentage>
  ''0%'' means the same as ''bottom'', ''100%'' means the same as ''top'', other values are linear interpolations of these. Negative values and values over 100% are interpreted as 0% and 100% respectively.

  Note that 100% minus the percentage corresponds to the initial position of the scrolling mechanism (if any) in case the content overflows.

For all other values, the content is aligned as for ''baseline''.

### Breaking grid elements across pages or columns

Grid elements may be broken across pages, columns or similar regions (including chained slots of another grid element), subject to the 'break-before', 'break-after' and 'break-inside' properties. In addition to the break points listed in the Fragmentation module [[!CSS3-BREAK]], a page break may occur between two rows in a template, if there is a possible break point at the same height or higher in all slots that span those two rows; and a page break may occur inside a row if there is a possible break point in all slots in that row.

Try to be more precise?

In the terminology of [[CSS3-BREAK]], a slot is a fragmenter of type ''region''.

A forced break on an element in a slot causes the rest of the flow of that slot to continue in another page, column or slot (depending on the type of break). The following cases are special:

- A forced page break causes all elements after the break (in document order) to be on a new page. On which subsequent page they fall depends on any page templates: there may not be a slot with the right name on the next page.

- A forced column break only has effect if the element's multicol ancestor is a descendant (is “inside”) the element's grid ancestor.

For example, this document fragment has a column element inside a grid element. Column breaks affect the content of a single slot, but do not affect the other slots of the grid element:

    <DIV STYLE="grid: 'ab' 'cd'">
     <DIV STYLE="flow: a">
      I'm in slot a.
     </DIV>
     <DIV STYLE="flow: b; columns: 20em">
      <P>This text is in columns.
      <H2 STYLE="break-before: column">Heading at top of column</H2>
      <P>...
     </DIV
    </DIV>

For example, this document fragment has a grid element inside a column element and thus the column break on the H2 is ignored:

    <DIV STYLE="columns: 20em">
     <DIV STYLE="grid: 'ab' 'cd'">
      <P>I'm inside slot a inside some columns.
      <H2 STYLE="break-before: column">No break</H2>
      <P>...
     </DIV>
    </DIV>

A slide presentation can be made with a template for each page (i.e., slide) and forced page break between the slides:

    @page	{ grid: "a" / 5em
    	        "@" / *
    	        "b" / auto }
    h1	{ page-break-before: always;
    	  flow: a }
    p.note	{ flow: b }

With a document similar to this: (fragment)

    <h1>Slide 1 title</h1>
    <ul>
     <li>Topic one
    </ul>
    <h1>Slide 2 title</h1>
    <ul>
     <li>More topics
    </ul>
    <p class=note>Note the note

The document in the example above doesn't have an element that corresponds to a slide; a slide simply starts at an H1 and ends before the next H1. But if there is a DIV around each slide (as is the case in many slide formats in practice), the same effect can also be achieved without page-based templates, by using the ''vh'' unit [[!CSS3VAL]]:

    div.slide {height: 100vh; grid: "a"/5em "*" "b"/intrinsic;
        page-break-before: always}
    h1 {flow: a}
    p.note {flow: b}

With a document similar to this: (fragment)

    <div class=slide>
     <h1>Slide 1 title</h1>
     <ul>
      <li>Topic one
     </ul>
    </div>
    <div class=slide>
     <h1>Slide 2 title</h1>
     <ul>
      <li>More topics
     </ul>
     <p class=note>Note the note
    </div>

### Stacking order

Slots are stacked in the order they are listed in 'grid-template' (order of their top left corners). 'z-index' applies. The general rules for stacking contexts [ref in CSS3?] apply.

This example uses 'z-index' and negative margins to make the middle slot partly overlap the other slots:

    body { grid: "a.b"
                 ".c."
                 "d.e";
           height: 240px;
           width: 240px }
    ::slot(a) { background: #0C0 }
    ::slot(b) { background: #C00 }
    ::slot(c) { background: #FD0; margin: -20px; z-index: 1 }
    ::slot(d) { background: #00C }
    ::slot(e) { background: #A0A }

Rendering of the above example.

### Floating elements inside templates

An element may be flowed into a slot and be a floating element at the same time. The following cases must be distinguished:

- Page-based floats – In paged media (see [[!CSS3PAGE]]), if the value of 'float' specifies that the element floats to the top or bottom of the page (in a horizontal writing mode) or the left or right of the page (in a vertical writing mode), the slot act as the page. (I.e., the element floats to the top, bottom, etc. of the slot, not of the page.
- Footnotes – Likewise, the footnote area for elements with 'float: footnote' is at the bottom of the slot, not of the page. The @footnote at-rule can position the footnote area at other places, such as the top, but always inside the slot.
- Normal floats – In other cases, the element floats normally within its containing block, which in this case is its slot in the template.

The '@footnote' at-rule from [[CSS3GCPM]] needs to be extended to apply to slots: '@footnote :first::slot(a)' is the footnote area of slot a on the first page.

## Page-based grid templates

A template can also be attached to a page, rather than an element. Such a template is called a page-based template as opposed to an element-based template.

The syntax of a page-based template is the same as that of an element-based one, but the declaration appears in an '@page' rule.

In a page-based template, the height and width are typically known (defined by the output media and the margin boxes, see [[CSS3PAGE]]). And content that overflows a slot typically is not clipped (or hidden behind a scrolling mechanism), but is continued on a subsequent page.

Because the grid template is not attached to an element but to a page, it is not scoped. The slot names are global, although they can be hidden to elements that have a grid ancestor that uses the same slot names.

This modifies the rules for 'flow' and 'grid-position': if these properties refer to a slot that is not found in a grid ancestor, they refer instead to a slot in a page template (if there is one and it has a slot of that name).

Similarly, a 'grid-position' that refers to grid lines by number, when set on an element that has no grid ancestor, refers to grid lines of the page template instead, if it exists. But unlike an element-based template, a page-based template is not automatically grown if 'grid-position' refers to a grid line that was not declared in the 'grid' property. Instead, 'grid-position' is ignored.

Should we simplify page-based templates to only allow <length> and * as row heights and column widths? (And treat illegal sizes as '*'?)

    @page :first {grid: "abc" "def"}
    @page {grid: "def"}

    body {flow: e}
    h1 {flow: b}

A page that has a grid template does not have a footnote area [[CSS3GCPM]]. Instead each slot in the grid template has its own footnote area.

If a slot of a page-based template on non-interactive media has an 'overflow' property of ''visible'', then content that overflows that slot in the block progression direction (i.e., below the slot in the case of horizontal text) causes a page break and is continued on the next page.

What happens in non-interactive media with an 'overflow' of ''scroll'' or ''auto''?

For page breaking purposes, each slot is considered as a page and the page break properties on the elements in that slot determine where the content for that slot is broken [[!CSS3-BREAK]]. Content after the page break is put in the slot of the same name on the next page that has such a slot. If there is no such page, the UA should display the content on a separate page.

Or: the content after the page break is not displayed? displayed in the default slot?

Note that this may happen if the template for the first page ('@page :first') uses a letter that occurs in no other @page rule. Possibly also if a page template is bound to a “named page” [[CSS3GCPM]] and that named page is not allowed to repeat. (At the time of writing, this is only a proposal in the GCPM Module.)

Note that an element A that appears later in the document than an element B may thus be displayed on an earlier page than element B, because their respective slots are broken across pages in different ways.

Because of limitations of a device (e.g., limited memory), it may be that a UA has to print a page (force page breaks) even though some slots aren't filled yet.

This example shows how the first page may have a different layout from the other pages. The slot ''a'' only occurs on the first page. If the content for that slot (in this case: all H1 elements) is too long, the remaining content of that slot will not be displayed. The slot ''@'' occurs on normal pages and all its content can thus be displayed by adding additional pages.

    @page :first {grid: "a" "*"}
    @page {grid: "*"}
    h1 {flow: a}

Note that “page masters” (sequences of different templates for sequences of pages) can be made with the selectors defined in [not yet decided].

Both page-based and element-based templates can be used in the same document.

    @page {grid: "a*"}
    :lang(fr} {flow: a}
    div.special {grid: "abc" "abd"}

Here is a page as one might find in a newspaper. It combines a layout template with multicolumn layout.

The front page of a newspaper, with the first parts of several stories that are continued on other pages and headings and images that span several columns.

    @page :first {
      grid: "A  A  A  A  A  A  A  A  A" / 5cm
    	".  .  .  .  .  .  .  .  ." / 0.25cm
    	"B  .  C  C  C  C  C  C  C" / *
    	"B  .  C  C  C  C  C  C  C" / *
    	"B  .  C  C  C  C  C  C  C" / *
    	"B  .  C  C  C  C  C  C  C" / *
    	"B  .  C  C  C  C  C  C  C" / *
    	"B  .  D  D  D  D  D  D  D" / *
    	"B  .  D  D  D  D  D  D  D" / *
    	"B  .  E  E  E  .  F  F  F" / *
    	"B  .  E  E  E  .  F  F  F" / *
    	"B  .  E  E  E  .  F  F  F" / *
    	 * 3em * 3em * 3em * 3em *
    }
    h1 {flow: a; border-bottom: thick; margin-bottom: 1.5em}
    #toc {flow: b; margin-right: -1.5em; border-right: thin;
      padding-right: 1.5em}
    #leader {flow: c; columns: 4; column-gap: 3em}
    #art1 {flow: d; columns: 4; column-gap: 3em; border-top: thin}
    #art2 {flow: e; columns: 2; column-gap: 3em}
    #art3 {flow: f; columns: 2; column-gap: 3em}

If a slot on a page is full and the content continues on the next page, it may be useful to insert something like “continued on page X.” This is useful at any page break, but more important if there are multiple “flows” of content on each page. Maybe a break-content property? 'break-content: "▶ continued on p. " targetcounter(???, page)' or extend text-overflow from [[!CSS3TEXT]]?

How do you set the 'vertical-align' property of a slot in a page? Does the '::slot()' pseudo-element apply? '@page :first :slot(A) {vertical-align: bottom}'

## Chaining slots: the 'chains' property

Slots must be rectangular, but the appearance of non-rectangular slots can be achieved to some extent by chaining slots together. Content that is positioned in the first slot of a chain is automatically continued in the second slot if the first slot is full, and then the third, etc.

Name: | chains
Value: | none | <letter>+ [ , <letter>+ ]*
Initial: | none
Applies to: | template elements
Inherited: | no
Percentages: | N/A
Computed value: | specified value

A value of ''none'' means the element's template has no chains. Otherwise the value consists of one or more comma-separated lists of letters. No letter may appear more than once in the value. Letters that do not occur in the template are ignored, but do not make the value invalid. A list with only ignored letters is itself ignored.

All the non-ignored letters in a list, except for the last one, must refer to slots whose size does not depend on their contents, otherwise the list is ignored. The size of a slot does not depend on its content if all the columns and all the rows that the slot spans have a width, respectively height, that is a <length> or '*'.

Each non-ignored list defines one chain.

Each chain is filled with content in an analogous way to the pages in paged media: all the content that is positioned to the first slot in the chain is flowed, in document order, into the first slot in the chain until the slot is full, the rest is flowed into the second slot until it is full, etc.

Content must only be split between slots at an allowed page break [[!CSS3PAGE]]. As for page breaks, if a break occurs in the margin between blocks, all adjoining margins are set to zero.

CSS3 does not define what is the best break point to split content over slots. However, it is recommended to use the last possible break point that does not cause overflow. (If such a break point exists.)

Note: It is the author's responsibility to make the height of relevant slots an integral number of lines if he wants to ensure that the lines in chained slots are aligned. The height does not automatically “snap” to a multiple of the line height.

The following template creates a double-staircase layout. In case the content is too long for the staircase, a slot of flexible height is added at the bottom.

    div { display: "@@..ff...." / 3.6em
                   ".aa..gg..." / 3.6em
                   "..bb..hh.." / 3.6em
                   "...cc..ii." / 3.6em
                   "....dd..jj" / 3.6em
                   ".........." / 0.6em
                   "eeee..kkkk" / auto;
       chains: @ a b c d e, f g h i j k}
    #first { position: @ }
    #second { position: f }

This could be applied to a document fragment such as

    <DIV>
      <P ID=first>...
      <P ID=second>...
    </DIV>

Here is a paragraph shaped as a circle:

    p { width: 12em;
        grid: ". . . . @ @ . . . ." / 1.2em
    	  ". . a a a a a a . ." / 1.2em
    	  ". b b b b b b b b ." / 1.2em
    	  ". b b b b b b b b ." / 1.2em
    	  "c c c c c c c c c c" / 1.2em
    	  "c c c c c c c c c c" / 1.2em
    	  ". d d d d d d d d ." / 1.2em
    	  ". d d d d d d d d ." / 1.2em
    	  ". . e e e e e e . ." / 1.2em
    	  ". . . . f f . . . ." / 1.2em
    	  "g g g g g g g g g g" / auto;
       chains: @ a b c d e f g }

Here is a page-based template that creates a two-column layout with a “hole” in the center:

    @page:first {
        grid:
        "aaaaaaa.ddddddd"
        "aaaaaaa.ddddddd"
        "aaaaaaa.ddddddd"
        "aaaaaaa.ddddddd"
        "bbbb.......eeee"
        "bbbb.ggggg.eeee"
        "bbbb.ggggg.eeee"
        "bbbb.ggggg.eeee"
        "bbbb.......eeee"
        "ccccccc.fffffff"
        "ccccccc.fffffff"
        "ccccccc.fffffff"
        "ccccccc.fffffff";
        chains: a b c d e f }
    @page::slot(g) {
        vertical-align: middle }

    body { flow: a }
    h1 { flow: g }

Note: For more analysis of the possibilities and limits of non-rectangular slots, see the PhD thesis of César Acebal [[ACEBAL2010]].

Chaining of rectangular slots is not enough to create layouts with holes, e.g., an image in the middle of a text. Allowing non-rectangular, connected regions (in addition to chaining) would allow an example such as this:

    grid: "A A A . . ."
          "A . A . . ."
          "A A A . . ."
          ". . . B B B"
          ". . . B . B"
          ". . . B B B";
    chains: a b;

Such cutouts in the middle of text usually create text that is difficult to read, and that is why there is no 'float: center', e.g. But the CSS WG is considering a new property 'wrap-flow' for absolutely positioned elements that would allow, e.g.: 'p {position: absolute; top: 1fr; left: 1fr; width: 1fr; height: 1fr; wrap-flow: both}' to absolutely position a P element on top of a template element and cause the content of the element under it to wrap around it as if it were a float [[CSS3-EXCLUSIONS]].

Note that a slot can have overflowing content even if it is part of a chain: it can have content that is too wide but cannot be broken.

The 'break-before', 'break-after' and 'break-inside' properties have values that control breaking of content between slots in a chain (in particular ''region'' and ''avoid-region'', see [[!CSS3-BREAK]]).

Note that, as defined in “Breaking grid elements across pages or columns” above, a slot is a fragmenter of type region in the terminology of [[!CSS3-BREAK]].

## CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent
  each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.
interoperable
  passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.
implementation
  a user agent which:
  1. implements the specification.
  2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
  3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

A test suite will be developed during the Candidate Recommendation phase of this specification.

## History

The following sections contain some of the use cases and design decision that led to this module and influenced its evolution.

### Developing a model of declaring, using, styling and resizing grids

(This section is not normative.)

The following types of use cases were considered for template-based layout.

1. Standard Web pages.

2. Grids and other table-like layouts. This includes grid layouts, frame layouts and table-like subdivision of a rectangular area.

3. A layout structure with “flex”ing information. The flexing is represented by constraints that specify how the cells are to relate to one another: which cells are to be allowed to grow or shrink and how much. There may also be a priority ordering, which determines, based on the size of the allowed display window, which cells shrink, which grow and under which conditions.

4. Layout structures with absolutely positioned (fixed-size) elements; for example a block of text into which several illustrations intrude at fixed positions within the block. This is like a float with respect to tightly wrapping the text around the intrusion, but the position of the intrusion is determined by the layout structure, not the content flowed into that structure.

   An example of this is a multicolumn layout with one or more “absolutely positioned floats” that intrude on the columns (see figure).

   An image (or a “pull quote”) is placed centered on the page and intrudes on two areas.

5. Multiple, disconnected, fixed-size areas on a page that are chained together, each one receiving the content that doesn't fit in the previous slot. In combination with page breaks, this may give a layout as often seen in newspapers: the first few lines of each story on the first page, the rest of the story in other areas on subsequent pages. (It will probably need a way to conditionally insert “continued on page 7” or similar text.)

For comparing proposals for template-based layouts, the working group identified four important aspects of each proposal:

1. the physical layout structures – the way of structuring the “cells” (slots) into which content is flowed. This includes a way to identify the various layout containers.

2. the binding mechanism – the way to specify that a given element (and its descendants) are to be placed in a given layout cell.

3. the property distribution mechanism – the way to put properties onto the layout structure and the cells within it.

4. the flexing mechanism – the way to describe how the layout structure should adapt itself to the higher level container (window) in which it is placed. This includes statements about which cells should grow and when they should grow.

In this specification, these aspects are as follows:

1. A character matrix is used to show the layout structure and the cells are named by the character used to show where they are positioned.

2. The binding of content to cells is handled by the 'position' property which identifies a cell to which the content is bound.

3. The shape, size and flexibility of the layout are specified with the character matrix. Some properties (background, border and vertical alignment) are attached to individual slots.

4. There is limited “flexing” information. The choice is between fixed size, a fraction of the available size or the content's intrinsic size. (The latter is further subject to min/max sizes specified on that content.) It is not possible to say, e.g., that some column can only become wider if all other columns are at their maximum size.

### Syntax, features and levels

(This section is not normative.)

The following is a partial list of design decisions and some arguments for and against each choice:

#### Named vs numbered slots

Named slots are very easy to understand and use. Experience with the draft showed that everybody who sees an example immediately understands what it means. Moreover, there is no need to use numbers or to count.

But it is different for absolutely positioned elements that use the grid. Those elements do not belong to a slot, they are merely placed on top of it, overlapping what is already there and each other.

Also, if grids are automatically extended with extra rows and columns (see below) based on content, then those extra rows and columns cannot have names and must (probably, see below) be referred to by number.

In this specification, named slots are used to create flows of content, while numbers are used for absolute positioning, although names can be used there too, if the author prefers.

#### Single letter names vs identifiers

The set of single letters is finite. And even if Unicode has many symbols, typing them may not be easy. Letters also do not permit to give a hint as to a slot's function (“heading,” “footer,” “advertisement”…).

On the other hand, any Unicode character can be typed as an escape, so if you run out of letters on the keyboard, you can always make a template like this:

    DIV {grid: "\1001\1002\1003"
               "\1001\1004\1005\1006"}
    DIV H1 {flow: \1001}

In practice, it is also hard to come up with meaningful identifiers and so single letters require less thinking. And if you always have single letters, you don't need spaces between them either, which makes big templates easier to read.

This specification therefore uses single letters (i.e., on 'grid-template'; the syntaxes of 'flow' and 'grid-position' are the same either way).

#### Extend 'display' or add a grid property

Grids can be defined on elements such as table cells, blocks, inline blocks or list items, but not on elements such as tables or, inline elements. That can be expressed in the syntax by adding the grid to the 'display' property: 'display: inline-block "abc" "abd"' is valid, but 'display: inline "abc" "abd"' would not be. Or it can be expressed by an implicit dependency between properties: 'display: inline-block; grid: "abc" "abd"' has a grid, but 'display: inline; grid: "abc" "abd"' ignores the grid. (In this case it is actually the computed value of 'display' that applies, so, e.g., if the element floats, it would be a block and thus have a grid.)

The Multicol specification uses the latter model: 'columns' is a separate property and only applies if the element is a block container. Given that precedent, it seemed reasonable to do the same here.

#### Combining columns and grids

Style rules can specify both a grid and columns for the same element:

    DIV {columns: 20em; grid: "aabccc"}

The specification could say that grids don't apply to column elements, or vice versa, or it could say that the columns apply to contents of the DIV, after it has been flowed into the default slot. This last option seemed the most useful. It is consistent with other elements that flow into a slots: they, too, can have columns.

#### Shorthand vs single property

It is convenient to specify the grid as the value of a single property. It puts the column and row sizes immediately next to the rows and columns they apply to. The typical column widths can either be omitted or are short values like '*' and '1em', and they fit easily in one line.

But if a grid has very many columns, or their sizes are complex (with many ''minmax()'' and ''calc()'' values, e.g.), it may be more readable to separate the slot names from the column sizes.

Also, separable properties for row and column sizes makes is easier to specify a transition/animation on them,, because you don't have to repeat the slot names (which cannot animate).

For those reasons, this specification defines the 'grid' property as a shorthand, at the cost for authors of having to learn four new properties instead of one (or zero, if the grid had been added to display (see above).

#### The number of rows and columns

If the grid template can also be specified with individual properties, how many columns & rows does it have if those properties contradict each other? Does the template (if not ''none'') determine the number of columns or the maximum of the template and the list of sizes? E.g.:

Specified values | Meaning if the template prevails | Meaning if the maximum value is used
--- | --- | ---
    grid-template: "a b c";
    grid-columns: * * * * *
    grid-template: "a b c";
    grid-columns: * * *
    grid-template: "a b c . .";
    grid-columns: * * * *

The former is consistent with the model for 'background-image' and 'background-size'. However, unlike for backgrounds, where a size without an image makes no sense, in this case a size without a slot name can make sense: it can represent empty space ('.') of the given size.

What is more likely: that people consciously specify empty rows and columns without putting dots in the template (to save typing or to avoid typing too many dots), or that people mistakenly add a size too many?

The specification currently sets the number of rows and columns to the maximum of the three properties.

#### '*' vs ''fr''

ISSUE-127: It is very common for the columns of a grid to have all the same width. That is because the slots in a grid, although of different widths themselves, typically are multiples of some fundamental measure. And thus equal columns is the default in the current syntax. And if it needs to be specified explicitly which columns have the same width (e.g., because there are also other columns in the grid) then the symbol for it is short: '*'. This is ideal for slots whose widths are small multiples of the fundamental measure. E.g., the widths of slots a, b and c in the following grid relate to each other as 1:3:2:

    grid: "a b b b c c"
          "a . . . c c"

However, if the ratios of the slots are ratios of larger numbers, the notation may become long. E.g., to make slots a and b with ratio 10:11, you would have to make 21 columns:

    grid: "aaaaaaaaaabbbbbbbbbbb"

It needs to be investigated if such ratios are frequent. (They don't seem to occur in printed magazines.) And if among the ratios that do occur, even if infrequently, there are some would be impractical to write in this way.

Assuming another notation is needed, it could be made by prefixing a number (e.g., '7* 10*') or, to make the notation look more like a dimension, by creating a special unit ''fr'' ('e.g., '3.5fr 5fr', which is the same as 7 + 10 stars).

If ''fr'' is added, it could either be in addition to '*' or instead of '*'. In the former case, '*' = '1fr'.

Unless it is shown that the notation with only '*' is not practical in real cases, it seems better to neither introduce numbers ('7*') nor units ('3.5fr'). It would give too many different ways to write the same grid, causing authors and readers of style sheets to have to think longer about each grid. (E.g., 'grid: "abbbcc"' could then also be written as 'grid: "abc" 1* 3* 2*' or 'grid: "abbbc" 0.5fr 0.5fr 0.5fr 0.5fr 1fr', etc.)

For the moment, the specification allows both the 'fr' and * notations.

#### Automatically add rows and columns

The main purpose of grids is to improve on absolute positioning for displaying elements in a different visual order. You typically know how many positions you need in advance.

If you don't need to change the visual order, then table layout can often align the elements sufficiently.

However, if table layout isn't possible (there are not enough elements in the source to create the necessary table rows, the control over the size of table cells is too limited, or you want a column-major table), it may be useful to use a grid in a similar manner to a table: the number of rows and columns is not specified in the style, but depends on the number of elements that are put in the grid.

E.g., you could transpose a table with an unknown number or rows like this:

    <TABLE>
     <TR><TD>A1 <TD>A2 <TD>A3
     <TR><TD>B1 <TD>B2 <TD>B3
     ...
     <TR><TD>K1 <TD>K2 <TD>K3
     ...
    </TABLE>

into a tabular display with an unknown number of columns somewhat like this:

    A1 B1... K1...
    A2 B2... K2...
    A3 B3... K3...

with style rules like this:

    TABLE {grid-columns: auto; grid-rows: auto}
    TD:first-child {grid-position: next / 1}
    TD {grid-position: same / next}

This uses keywords instead of numbers, as described in “Automatic placement of elements” below.

What is the size of the added rows and columns? The example above assumes the size of the single, specified grid cell is simply repeated. There could also be a marker in the list of sizes to indicate which set of sizes is repeated. Or there could be a separate property with the list of sizes that is repeated for all added rows and columns.

It would probably be difficult to specify that any number of columns of size A can be added, but that the last column must have size B…

The current specification specifies that extra rows and columns are added when needed for absolute positioning ('grid-position'). It is not possible to automatically create new, anonymous flows. (But sometimes you can use multi-column elements to do that, and possibly grid templates can be attached to columns, via a '::column') pseudo-element, see [[CSS3GCPM]].)

#### Rectangular slots vs non-rectangular slots

Non-rectangular slots, such as

    grid: "a a a"
          "a a ."
          "a . ."

are probably something for level 4. Or maybe the Exclusions module can be used instead.

#### Chained slots

Many layouts found in magazines can be handled with multi-columns [[CSS3COL]] (possibly extended with ideas in GCPM [[CSS3GCPM]]) or with multi-column elements inside a grid, but some more complicated designs appear to require chained slots (and some would be easier to specify that way than with columns).

    grid: "A A E"
          "A A E"
          "B B E"
          "C D E";
    chains: B C D E

Also, when grids are applied to pages (see “Page grids” below), it is essential that slots are connected to slots on the next or some subsequent page, so that the overflow can find a place.

But this could be a feature for level 4.

#### Page grids

Typically in magazines, each page has a slightly different layout of slots, although they are all based on the same set of columns, usually between 3 and 6. It is difficult to specify the grids of all pages as a single long grid on the BODY element; and indeed impossible if you don't know how many pages use each kind of layout.

Applying grids to '@page' is an alternative. It has a few issues:

- We need named paged ('@page news {…}'), a way for an element to say it needs to start on a specific kind of page ('break-before: page(news)' or 'break-before; always; page: news'), and a way to say which style follows which ('@page news-odd {next: news-even}').
- What happens to footnotes [[CSS3GCPM]]? Each slot probably has its own footnote area.
- How are slots on different pages connected? Probably by having the same name.

This is probably for level 4.

#### Style the slots (pseudo-elements)

Slots can have properties of their own: background, overflow, writing-mode, direction, box-shadow, margin, border, padding. What else?

For 'vertical-align', see “Vertical alignment inside slots” below.

This specification also allows 'z-index', because then you can give a slot negative margins and have it partially overlap some other slot.

'Wrap-flow' (from CSS Exclusions and Shapes) could be useful for slots with negative margins, too.

'Content' might be useful, but is maybe too powerful: you could write a whole document, with proper styling, in a style sheet and would only need an empty source document.

#### Style the contents of slots (region-based styling)

By means of a new pseudo-element, style could be applied to only the part of an element that is inside a certain slot (similar to how ''::first-line'' applies style only to the part of an element in the first line of a paragraph):

    P::fragment(a) {font-size: large}

This looks like a level 4 feature.

#### Orient the grid with writing mode vs orient the slots

A grid can contain vertical text, but the grid itself probably doesn't need to be rotated or flipped. Indeed, it would be confusing if it did. In other words, the following element has vertical text in the top right slot and an image in the bottom right slot:

    <DIV STYLE="grid: "a*" "ab"; writing-mode: vertical-rl">
     Some text here...
     <IMG STYLE="flow: b"...>
     ...
    </DIV>

In this specification, a grid template is always laid out with the first row at the top and the first column on the left. But the slots themselves can have a writing mode (by default the one “inherited” from the grid element).

#### Indicating the default slot ('*' vs '@')

The symbol '*' seems a good choice, as the concept of default is somewhat similar to the concept of wildcard. The '*' is also used to set the size of rows and columns, but the context is probably enough to avoid confusion.

#### Flowing vs absolutely positioning content in a grid

The most flexible model for positioning content in a grid is to consider the slot as a flow, which can contain zero, one or more elements and even anonymous content. This is the same concept as a table cell: the cell also is a separate flow that can contain several elements as well as anonymous content.

An element inside a slot thus has its size determined as an element in normal flow. E.g., if you set a background on it, the background covers the element only, not the whole slot. (But the slot has its own background property for that.)

But you may also want to use the grid as a coordinate system for absolutely positioned elements, which then overlap with the contents of the grid (above or below it, depending on 'z-index').

And, as “Automatically add rows and columns” above explains, you may want a table-like display of elements that are neither absolutely positioned nor flowed, but the table properties aren't powerful enough.

Those two ideas can be added to the grid template model: the declaration of the grid is reused, but instead of using 'flow' to position an element into it, the element is absolutely positioned on top of it. But unlike the absolute positioning in level 2, this positioning is allowed to influence the size of the grid element on which it is positioned.

There are two ideas for how to do this. One idea is to define a special kind of unit, ''gr'', which is only defined for elements that have a grid ancestor, and which can be used on the 'top', 'right', 'bottom' and 'left', properties of absolutely positioned elements. ''left: 2.25gr'' means the position is in the third column, one quarter of the way towards the fourth column.

Another way is to define a new property 'grid-position' (shorthand for 'grid-position-x' and 'grid-position-y'), which, if set, overrides 'top', 'left', etc. and which takes numbers or names of slots (up to four of them, to set the four sides).

Yet another way is to extend 'top', 'left', etc., with unitless values (meaning the n'th grid column or row) and letters (meaning the appropriate edge of the slot of that name).

This specification adds a 'grid-position' property, which overrides 'top', 'right', etc. When it is used, it also indicates that the element takes part in the size calculations of the rows and columns it is put on top of.

#### Use the pseudo-class to create additional slots

If you want a flow that overlaps with a grid element (i.e., absolutely positioned on top of it), but that flow doesn't correspond to an element in the document you could create a pseudo-element that is like an “absolutely positioned slot:”

    DIV::slot(z) {position: absolute; grid-position: 2 / 2}

Like other '::slot()' pseudo-elements, it applies to grid elements only (the DIV in this example must be a grid element), but unlike other '::slot()' pseudo-elements it doesn't style an existing slot, but implicitly creates a new one. (It is some kind of error if the name ''z'' already exists in the grid template.)

Then you can flow other elements into this slot:

    .note {flow: z}

This mechanism is pretty difficult to understand. Experience shows it is also difficult to explain and in a style sheet it is difficult to recognize. What are the use cases (that can't be done with negative margins, absolute positioning, etc.)? This specification does not allow the creation of arbitrary, absolutely positioned pseudo-elements. If needed, it could be added in level 4.

#### Names for edges

If you position elements absolutely (with 'top', 'left', etc. or with 'grid-position', see “Flowing vs absolutely positioning content in a grid” above), you can use numbers to refer to grid lines: from '1' (the left edge of the first column) to N+1 (the right edge of the last column) and ditto for rows. You can also refer to the edges of slots, for an extra level of indirection and to avoid the need for numbers.

It has been suggested to create a mechanism to assign arbitrary names to grid lines, even multiple aliases for the same grid line. You could then absolutely position an element at grid line ''foo'' (e.g., 'grid-position: foo / 1') and later assign that alias ''foo'' to a different grid line without having to change the style rules that use it. E.g., the following could make the name ''a-line'' an alias for the number 2 and ''b-line'' an alias for the number 4:

    /* Create a 7x2 grid template */
    DIV {grid-columns: * a-line * * b-line * *; grid-rows: * *}
    /* Put H2 at cell x=4, y=2 */
    DIV H2 {position: absolute; grid-position: b-line / 2}

(This could also use strings instead of identifiers, for even more flexible names.)

This turns out to be very difficult to explain to users, partly because of the syntax, partly because of the indirection, which comes on top of the inherent indirection that CSS already requires (the style rules are in a style sheet, not on the elements they apply to) and the indirection of specifying a position indirectly by referring to an abstract grid.

Also, the number of usages of 'grid-position' (for a given grid) is not likely to be so big that a symbolic name for a grid line is necessary. Moreover, this is easy to handle with a macro processor.

This specification does not include a mechanism to assign aliases to grid lines.

#### Position from the right and bottom

If elements are absolutely positioned relative to a grid (with 'top', 'left' or 'grid-position', see “Flowing vs absolutely positioning content in a grid” above), and especially if the grid can automatically grow (see “Automatically add rows and columns”) it might be useful to position elements relative to the last row or column, independent of how many rows or columns the grid element has. That could be done with negative numbers:

    grid-position-x: -1

This specification does not allow positioning from the end. If uses cases are found, it could be added in level 4.

#### Syntax features to shorten long grid specifications

Grids are typically very regular (see also '*' vs ''fr'' above). E.g., if you specify the margins between slots in the template (rather then with a 'margin' property on the slots), you often end up with a grid similar to this:

    grid-template: "a.b.c.d.e.f";
    grid-columns: * 1em * 1em * 1em * 1em * 1em *

Even for the columns of broadsheet newspapers (6–8 columns), that still fits easily on one line, even with margins between the columns. But if the grid gets even longer, there may be a need for abbreviations, e.g.:

    grid-columns: * repeat(1em *, 5)

or

    grid-columns: * (1em *)[5]

or

    grid-columns: * 5//1em *//

This makes the syntax harder to learn and read, though. So it needs some good use cases first. This can be postponed to level 4.

#### ''fit-content'' vs ''auto'' vs…

Column sizes can have an exact size or a minimum and maximum size, by means of ''minmax()''. A common case is probably ''minmax(min-content, max-content)''. (Is that really true?) Should that common case have its own keyword? If so, should that keyword be ''fit-content''?

''fit-content'' in the Box Module is defined as something else, viz., min(max(available width, ''min-content''), ''max-content'')).

In this specification, ''auto'' (not ''fit-content'') is allowed for column and row sizes and has the meaning of ''minmax(min-content, max-content)''.

#### Vertical alignment inside slots

The content of a slot can be vertically aligned with 'vertical-align', exactly like the content in table cells. (Despite the name, it would, of course, be horizontal alignment if the slot's writing mode was vertical.)

There could also be a property that applies exclusively to grids (grid-slot-align?), just like Flexbox proposes an alignment property that applies exclusively to flex items ('flex-item-align'). But why should users learn a new property when 'vertical-align' fits so well?

Looking beyond tables, flex boxes and grids, there is also a need for vertical alignment of content in normal blocks (when those blocks have a height that does not depend on their contents). There might thus also be a new property that applies uniformly to all cases. E.g.: ''block-align: top | bottom | middle | baseline | <percentage> | <length>'', where ''baseline'' simply means ''top'' everywhere outside grids and tables. (A separate question is if it would override 'vertical-align' in tables). Or 'vertical-align' itself could be generalized to apply to all block container boxes (although the meaning would be ambiguous on ''inline-block'').

Given how little progress has been made on this generally applicable property, the present specification just reuses 'vertical-align'. If a differently named property is developed after the grid template module becomes a REC, then whatever interaction it has with 'vertical-align' would apply to slots, too.

#### Horizontal alignment inside slots

Elements can be centered in, or aligned to a side of, their parent with ''auto'' margins. But this doesn't always work: for many combinations of 'width' and 'direction', the auto margins are ignored. There may thus be a new property ('block-align: center'?) or value ('margin: any'?) in the future for correct alignment and centering.

Or should there be an alignment property that only applies to grid slots? 'grid-slot-align'?

This specification defines nothing, in the expectation of a general solution in the future.

#### Auto margins on slots

Do ''auto'' margins on slots (e.g., ''::slot(a) {margin: auto}'') do anything? Or are they simply 0?

#### Page floats

[[CSS3GCPM]] defines “page floats” (which are slightly misnamed, because they cause floats to float not just to the top or bottom of a page, but also to the top or bottom of a column in certain contexts). What do they do in slots?

This specification treats a slot similar to a column, e.g., ''float: top'' means the top of the slot, not of the page. Similarly, if slots are chained, then the float modifiers select the next slot, not the next page.

#### Break-* properties

The 'break-before', 'break-after' and 'break-inside' properties in the draft of February 2012 (see [[CSS3-BREAK]]) have keywords ''region'' and ''avoid-region''. Should they be called ''slot'' and ''avoid-slot'' instead?

#### Automatic placement of elements

There may be cases where elements are to be placed based on where the previous elements was placed, which isn't always expressible with a clever selector. An example is placing DD elements in the same slot as the preceding DT element. (Note that there may be more than one DD associated with a single DT, so a selector such as ''DT + DD'' doesn't work. The selector ''DT ~ DD'' doesn't work either, because it selects DDs that belong to the next DT, too.).

A keyword may help: 'flow: same'.

In some cases, elements may need to be placed in the next column or row after the previous element. If there are cases where rows or columns need to be added automatically based on content, those are probably also the cases where element are to be placed in the “next” row or column w.r.t. to the preceding element.

E.g., with a keyword: 'grid-position: next/same' (for “next column, same row”).

This specification defines ''same'' on 'flow' and 'grid-position' and ''next'' on 'grid-position' only, because so far there are no use case for ''next'' on 'flow'.

## Changes

Summary of major changes since the 29 November 2011 draft:

- Added a note about accessibility.
- Instead of directly on 'display', the grid template is now specified with a new property, 'grid', to be consistent with 'columns' in the multi-column specification [[CSS3COL]], and also to allow the grid to be a shorthand for more primitive properties: 'grid-template', 'grid-rows' and 'grid-columns', which makes animations easier to specify.
- Replaced @ by * to indicate the default slot. The asterisk looks better and may also be easier to remember, because of its association with the notion of “wild card.”
- Added dimensions with the ''fr'' unit as an alternative to * on row and column sizes (and an issue about whether these units are useful enough).
- Removed the ''gr'' unit as a means to absolutely position elements on a grid and instead added a special property, 'grid-position' (shorthand for 'grid-position-x' and 'grid-position-y').
- Absolutely positioned elements, if positioned relative to a grid, are now taken into account when calculating the size of the grid.
- Because of the above, it is now possible to automatically add rows and columns to a grid element if elements are absolutely positioned relative to grid lines beyond those declared in the grid template.
- Moved the 'chains' property to level 4 of this module. (In theory, it shouldn't be more difficult for a UA to balance content in a chain of slots than in a chain of columns, but it requires some new code, nevertheless.)
- Moved page-based templates ('@page') to level 4. The scope of slot names, automatic chaining of slots on different pages, and especially the question of how to create more than three page templates (':first', ':left' and ':right') need more investigation.
- Moved region-based styling (i.e., pseudo-element selectors for styling elements differently based on which slots they fall into) to level 4. It is only useful in combination with chaining of slots (the 'chains' property) or page-based templates.
- Renamed the keyword ''fit-content'' to ''auto''. The former has a specific meaning in [[CSS3BOX]], which is related, but not the same.
- The margin properties are now allowed on slots. Even though this is redundant with the possibility to create empty slots in the grid template, it seems more consistent, because slots also have borders (which, unlike in tables, do not collapse).
- Different set of constraints and a different function to optimize when calculating the size of rows and columns: the row heights and column widths are now considered together, which should guarantee that the overall size of the grid element is as small as possible in all cases and also makes the handling of width and height symmetric.
- References to 'page-break-*' properties and to [[CSS3PAGE]] have been replaced with references to 'break-*' properties and the new [[CSS3-BREAK]] module.
- Now allows <percentage> on 'vertical-align'.
- Generalized the text for 'vertical-align' to apply to all writing modes.

## Acknowledgments

The first ideas for describing a template in CSS date from 1996 and are described in Frame-based layout via Style Sheets by Bert Bos, Dave Raggett and Håkon Wium Lie. The idea was revived in 2005 on the request of W3C's Device Independence Working Group and benefited especially from discussions with Rhys Lewis and Rotan Hanrahan from that group.

This specification was further influenced by ideas about form layout by Dave Raggett [member-only link] and an early write-up of the features of XUL by Ian Hickson [member-only link].

Andy Clarke, Jina Bolton and Kevin Lawver provided use cases, examples and requirements. The analysis in the History section is a slightly shortened version of work by Steve Zilles.

César Acebal built the first prototype. Andrew Fedoniouk built the second. A third prototype was made by Alexis Deveria.

## References

Normative references:

[Here will be inserted the file “normative.inc”]

Other references:

[Here will be inserted the file “informative.inc”]

## Index

## Property index