# CSS Scroll Snapping Change Proposal

## Unofficial Proposal Draft, 2 August 2016

This version:
  https://drafts.csswg.org/css-scroll-snap/
Issue Tracking:
  GitHub
  Inline In Spec
Editors:
  Tab Atkins-Bittner (Google)
  Elika J. Etemad / fantasai (Invited Expert)

Copyright © 2016 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and document use rules apply.

## Abstract

A brief description of an alternate model for scroll-snapping.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.

## Status of this document

The following features are at-risk, and may be dropped during the CR period:

- point value of scroll-snap-type

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

## 1. Introduction

We think scroll snapping is a great idea, and fully support exposing this functionality through CSS. However, a major weakness of the current spec is the way it conceives snapping on a coordinate model rather than a box model. This requires a lot of manual calculations in figuring out the correct coordinates from the box model; and also makes sensible scroll-snap settings dependent on the relative sizes of the viewport and the snappable contents, causing problems for users are unexpectedly large and/or small screens (a problem commonly ignored by many authors).

This proposal builds off of roc’s model, using an area-snapping model to intelligently handle adaptation to multiple screen sizes. It also adds group alignment as a built-in concept, rather than requiring authors to build one in JavaScript.

## 2. Use Cases

Use Case 1: Snapping to the start or middle of each box e.g. address book (start) or photo album (middle)

1. Snapping to 0.25rem above the top of each heading

       :root { scroll-snap-type: proximity; }
       h1, h2, h3, h4, h5, h6 {
         scroll-snap-align: start;
         scroll-snap-margin: 0.25em;
       }

2. Snapping to the center of each photo

       :root { scroll-snap-type: mandatory; }
       img { scroll-snap-align: center; }

Use Case 2: Snapping to boxes (or points) in 2D e.g. on a map, where you want to snap points of interest to the center, or a flow-chart diagram, where you want to snap the edges of each box into the visible area. In both cases, you don’t want objects wholly outside the visible area to influence snapping.

1. Snapping each flow chart entry to within the viewport when it falls near the edge:

       :root {
         scroll-snap-type: proximity;
       }
       li {
         scroll-snap-align: start;
       }

2. Snapping each city on a map to the center of the viewport, but only once it gets near the center in both dimensions:

       :root {
         scroll-snap-type: proximity;
       }
       .city {
         scroll-snap-align: center;
       }

Use Case 3: Slideshow, where successive slides are arranged horizontally, and sometimes "detail" slides are placed below the "main" slide for that point.

    <div class="slides">
      <div class="slide">...</div>
      <div class="slide">...</div>
      <div class="slide details">
        <div class="slide">...</div>
        <div class="slide">...</div>
      </div>
      <div class="slide">...</div>
    </div>
    <style>
    .slides {
      display: flex;
      flex-flow: row;
      scroll-snap-type: mandatory;
      overflow-x: scroll;
      width: 100vw;
      height: 100vh;
    }
    .slide {
      scroll-snap-align: start;
      width: 100vw;
      min-height: 100vh;
    }
    .slide.details {
      display: flex;
      flex-flow: column;
      scroll-snap-type: mandatory;
      overflow-y: scroll;
    }
    </style>

## 3. Overview of Change

On the scroll container:

Spec | Proposal | Priority
--- | --- | ---
scroll-snap-type: none | mandatory | proximity | scroll-snap-type: none | [ mandatory | proximity ] || [ x | y | block | inline | both | point] | High priority
''scroll-snap-destination: <position>'' | ''scroll-snap-padding: [ <length> | <percentage> ]{1,4}'' |

On the children:

Spec | Proposal | Priority
--- | --- | ---
''scroll-snap-coordinate: <position>#'' | scroll-snap-align: [ none | start | end | center ]{1,2} | High priority
n/a | ''scroll-snap-margin: <length>{1,4}'' | High priority

## 4. Scroll Snapping Model

This module introduces control over scroll snap positions, which are scroll positions that produce particular alignments of content within a scrollable viewport. Using the scroll-snap-type property on the relevant scroll container, the author can request a particular bias for the viewport to land on a valid snap position after scrolling operations.

Snap positions are specified as a particular alignment (scroll-snap-align) of a box’s scroll snap area (its border bounding box, as modified by scroll-snap-margin) within the scroll container’s snapport (its scrollport, as reduced by scroll-snap-padding). This is conceptually equivalent to specifying the alignment of an alignment subject within an alignment container. A scroll position that satisfies the specified alignment is a valid snap position.

The act of adjusting the scroll position of a scroll container’s scrollport such that it is aligned to a snap position is called snapping, and a scroll container is said to be snapped to a snap position if its scrollport’s scroll position is that snap position and there is no active scrolling operation. The CSS Scroll Snap Module intentionally does not specify nor mandate any precise animations or physics used to enforce snap positions; this is left up to the user agent.

Snap positions only affect the nearest ancestor scroll container on the element’s containing block chain.

## 5. Capturing Scroll Snap Areas: Properties on the scroll container

### 5.1. Scroll Snapping Rules: the scroll-snap-type property

Name: | scroll-snap-type
Value: | none | [ proximity | mandatory ] || [ x | y | block | inline | both | point ]
Initial: | none
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | as specified
Animatable: | no

The scroll-snap-type property specifies whether a scroll container is a scroll snap container, how strictly it snaps, and which axes are considered.

We’re considering splitting this into subproperties. Current proposed names are scroll-snap for the current grammar, scroll-snap-affinity for the proximity/mandatory distinction, and scroll-snap-axis for the x/y/etc distinction.

The strictness values (none, proximity, mandatory) specify how strictly snap positions are enforced on the scroll container (by forcing an adjustment to the scroll position). Values are defined as follows:

none
  If specified on a scroll container, the scroll container must not snap: all scroll positions are equally valid.

  If specified on a non-scroll container, this value has no effect.

proximity
  If specified on a scroll container, the scroll container may snap to a valid snap position at the termination of a scroll, at the discretion of the UA given the parameters of the scroll.

  If specified on a non-scroll container, this value “traps” descendant boxes’ snap positions, preventing them from affecting any ancestor scroll containers.

mandatory
  If specified on a scroll container, the scroll container is required to be snapped to a valid snap position when there are no active scrolling operations. That is, it must snap to a valid snap position at the termination of a scroll, if any such exist. (If none exist, then no snapping occurs.)

  If specified on a non-scroll container, this value “traps” descendant boxes’ snap positions, preventing them from affecting any ancestor scroll containers.

A box captures snap positions if it is a scroll container or has a value other than none for scroll-snap-type. If a box’s nearest snap-position capturing ancestor on its containing block chain is a scroll container with a non-none value for scroll-snap-type, that is the box’s scroll snap container. Otherwise, the box has no scroll snap container, and its snap positions do not trigger snapping.

Authors should use mandatory snap positions with consideration of varyingly-sized screens and (if applicable) varying-sized content. In particular, although access to snapped elements larger than the viewport is handled by the UA, if authors assign mandatory snapping to non-adjacent siblings, content in between can become inaccessible in cases where it is longer than the screen.
For example, if an author wishes to force snapping to the top of each section heading, s/he could accomplish this in two ways: snapping the headings

    h1, h2, h3, h4, h5, h6 { scroll-snap-align: start; } /* snap headings - but not section content */

or snapping the section elements.

    section { scroll-snap-align: start; }                /* snap entire section - including content */

If the author chooses mandatory snapping of the headings, and one section is longer than the viewport, then the reader will have difficulty accessing the content that overflows the screen, because mandatory snapping does not allow the scroll position to rest on the content between the snapped headings.

However, if the author chooses mandatory snapping of the section element (which contains all the content of the section) then the UA can allow the reader to scroll freely through the entire section in the cases where the content is longer than the screen.

The axis values specify what axis(es) are affected by snap positions, and whether snap positions are evaluated independently per axis, or together as a 2D point. Values are defined as follows:

x
  The scroll container axis-snaps to snap positions in its horizontal axis only.
y
  The scroll container axis-snaps to snap positions in its vertical axis only.
block
  The scroll container axis-snaps to snap positions in its block axis only.
inline
  The scroll container axis-snaps to snap positions in its inline axis only.
both
  The scroll container axis-snaps to snap positions in both of its axises independently (potentially snapping to different elements in each axis).
point
  The scroll container point-snaps to snap positions in both axises simultaneously, treating each element’s snap position as a single 2D position (rather than potentially snapping to different elements in each axis).

If no axis value is specified, then the axis is automatically computed:

- If the scroll container is only scrollable in one axis (only one axis has its overflow set to auto or scroll) it axis-snaps in the scrollable axis only.

- Otherwise, it axis-snaps in its block axis only.

If the content or layout of the scroll container changes (e.g. content is added, moved, deleted, resized), the UA must re-evaluate, and potentially re-snap if necessary, the resulting scroll position once the positions of the content have restabilized. This can result in no snapping in some cases (e.g. if there are no nearby snap positions for a proximity-snapping scroll container). However, if there was a previously-snapped snap position (associated with the same element) that still exists after such changes, the UA must remain snapped to it—even if the changes would have placed a different snap position closer to the current scroll position. Otherwise, the scroll container must be re-snapped as if the user had scrolled to its current scroll position (as an explicit scroll, if no other scroll operation is active).

### 5.2. Scroll Snapport: the scroll-snap-padding property

Name: | scroll-snap-padding
Value: | [ <length> | <percentage> ]{1,4}
Initial: | 0
Applies to: | scroll containers
Inherited: | no
Percentages: | relative to the corresponding dimension of the scroll container’s scrollport
Computed value: | as specified, with lengths made absolute
Animatable: | as length, percentage, or calc

The scroll-snap-padding property defines the scroll snapport—the area of the scrollport that is used as the alignment container for the scroll snap areas when calculating snap positions. Values are interpreted as for padding, and specify inward offsets from each edge of the scrollport.

In this example, points of interest in a map are centered within the portion of the viewport that does not include the toolbar overlay.

    map {
      overflow: scroll;
      scroll-snap-type: proximity;
      scroll-snap-padding: 3em 0 0 0;
    }
    toolbar {
      position: absolute;
      margin: 0.5em;
      top: 0; left: 0; right: 0;
      height: 2em;
    }
    city {
      scroll-snap-align: center;
    }

This property is a shorthand property that sets all of the scroll-snap-padding-* longhands in one declaration.

## 6. Aligning Scroll Snap Areas: Properties on the scrolling content

### 6.1. Scroll Snapping Margin: the scroll-snap-margin property

Name: | scroll-snap-margin
Value: | <length>{1,4}
Initial: | 0
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | as specified, with lengths made absolute
Animatable: | as length

The scroll-snap-margin property defines the scroll snap area that is used for snapping this box to the viewport. The <length> values give outsets (interpreted as for margin or border-image-outset). The scroll snap area is the rectangular bounding box of the transformed border box, plus the specified outsets, axis-aligned in the scroll container’s coordinate space.

Note: This ensures that the scroll snap area is always rectangular and axis-aligned to the scroll container’s coordinate space.

This property is a shorthand property that sets all of the scroll-snap-margin-* longhands in one declaration.

### 6.2. Scroll Snapping Alignment: the scroll-snap-align property

Name: | scroll-snap-align
Value: | [ none | start | end | center ]{1,2}
Initial: | none
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | two keywords
Animatable: | no

The scroll-snap-align property specifies the box’s snap position as an alignment of its snap area (as the alignment subject) within its snap container’s snapport (as the alignment container). The two values specify snapping alignment in the inline axis and block axis, respectively. If only one value is specified, the second value defaults to the same value.

The following example aligns the start edge of the box’s snap area to the start edge of the scroll container’s snapport:

    section { scroll-snap-align: start; }

The following example aligns the center of each city to the center of the scroll container’s snapport, snapping only when the city is centered in both axes:

    .map { scroll-snap-type: proximity point; }
    .map .city { scroll-snap-align: center; }

The following example aligns the center of each photo to the center of the scroll container’s snapport, snapping independently in each axis:

    .photos { scroll-snap-type: mandatory both; }
    img { scroll-snap-align: center; }

Values are defined as follows:

none
  This box does not define a snap position in the specified axis.
start
  Start alignment of this box’s scroll snap area within the scroll container’s snapport is a valid snap position in the specified axis.
end
  End alignment of this box’s scroll snap area within the scroll container’s snapport is a valid snap position in the specified axis.
center
  Center alignment of this box’s scroll snap area within the scroll container’s snapport is a valid snap position in the specified axis.

If the element’s scroll container is point-snapping, and this property does not specify a valid snap position in both axises (that is, it contains none), the element does not contribute any snap positions at all.

For all of these values, the block or inline axis is relative to the element’s parent’s writing mode.

Is this the correct writing mode to compute against? Or should it be the scroll container’s writing mode?

Why no <length> or <position> values?

The values here represent alignments (in the sense of align-self and justify-self), so are consistent with that syntax. We chose to use this simpler syntax without lengths or percentages because the scroll-snap-margin concept already provides length offsets—but does so in a smarter way, that degrades better on small screens (see above) because it provides more information (a box, rather than a point) to the UA. We could have also added lengths here, but it would provide multiple ways to do the same thing, which is additional overhead for implementation, testing, and (most importantly) author learning. It also introduces more room for cascading errors, and guides authors in the wrong direction—away from scroll-snap-margin.

#### 6.2.1. Scoping Valid Snap Positions to Visible Boxes

Since the purpose of scroll snapping is to align content within the viewport for optimal viewing: in all cases, the specified alignment creates a valid snap position only if at least part of the snap area is within the snapport. For example, a snap area is top-aligned to the snapport if its top edge is coincident with the snapport’s top edge; however, this alignment is nonetheless not a valid snap position if the entire snap area is outside the snapport.

Why limit snapping to only when the element is visible?
As the WebKit implementers point out, extending a snap edge infinitely across the canvas only allows for snapping gridded layouts, and produces odd behavior for the user when off-screen elements do not align with on-screen elements. (If this requirement is onerous for implementers however, we can default to a gridded behavior and introduce a switch to get smarter behavior.)

#### 6.2.2. Snapping Boxes that Overflow the Scrollport

If the snap area is larger than the snapport in a particular axis, and there are no other snap areas within the snapport that would provide a snap position aligning the overflowing snap area within the snapport, then any scroll position in which the snap area covers the snapport is a valid snap position in that axis. The UA may use the specified alignment as a more precise target for certain scroll operations (e.g. inertial scrolling or explicit paging).

For example, take the third code fragment in the previous example, which had a photo as the area. The author wants mandatory snapping from item to item, but if the item happens to be larger than your viewport, you want to be able to scroll around the whole thing once you’re over it.

Since the snap area is larger than the snapport, while the area fully fills the viewport, the container can be scrolled arbitrarily and will not try to snap back to its aligned position. However, if the container is scrolled such that the area no longer fully fills the viewport in an axis, the area resists outward scrolling until you fling out or pull it sufficiently to trigger snapping to a different snap position.

#### 6.2.3. Unreachable Snap Areas

If a snap position is unreachable as specified, such that aligning to it would require scrolling the scroll container’s viewport past the edge of its scrollable area, the used snap position for this snap area is the position resulting from scrolling as much as possible in each relevant axis toward the desired snap position.

### 6.3. Scroll Snap Limits: the scroll-snap-stop property

Name: | scroll-snap-stop
Value: | normal | always
Initial: | normal
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | as specified
Animatable: | no

This property specifies whether the snap position absorbs all remaining inertia during an inertial scroll, or allows the inertial scroll to pass multiple snap positions before coming to rest. Values are defined as follows:

normal
  A snap position defined by this element does not interfere with the inertia of an inertial scroll that is passing across it, unless it is the landing snap position.
always
  A snap position defined by this element, when encountered by an inertial scroll, absorbs all remaining inertia from an inertial scroll, forcing a stop at this snap position, exactly as if the scroll had enough inertia to reach the snap position, but not enough to escape it.

  Note: This means that if all snap positions in a scroller have scroll-snap-stop: always, an inertial scroll can only move one snap position per inertial scroll action.

## 7. Snapping Mechanics

The precise model algorithm to select a snap position to snap to is intentionally left mostly undefined, so that user agents can take into account sophisticated models of user intention and interaction and adjust how they respond over time, to best serve the user.

This section defines some useful concepts to aid in discussing scroll-snapping mechanics, and provides some guidelines for what an effective scroll-snapping strategy might look like. User agents are encouraged to adapt this guidance and apply their own best judgement when defining their own snapping behavior. It also provides a small number of behavior requirements, to ensure a minimum reasonable behavior that authors can depend on when designing their interfaces with scroll-snapping in mind.

### 7.1. Types of Scrolling Methods

When a page is scrolled, the action is performed with an intended end position and/or an intended direction. Each combination of these two things defines a distinct category of scrolling, which can be treated slightly differently:

explicit scrolling

  A scroll is explicit if it has an intended end position, but no intended direction.

  This includes methods such as:

  - a panning gesture, released without momentum

  - manipulating the scrollbar "thumb" explicitly

  - programmatically scrolling via APIs such as `scrollTo()`

  - tabbing through the document’s focusable elements

  - navigating to an anchor within the page

inertial scrolling

  A scroll is inertial if it has both an intended end position and an intended direction.

  This includes methods such as:

  - a "fling" gesture, released with momentum (the "intended" end position might be implicitly determined by the UA’s scrolling physics, but the strength of the user’s fling still expresses a weak intention about where the scroll should end up)

  - a mousewheel scroll

  - programmatically scrolling via APIs such as `scrollBy()`

  The scroll position that an inertial scroll would naturally land on without further intervention is the natural end-point.

directional scrolling

  A scroll is directional if it has an intended direction, but no intended end point.

  This includes methods such as:

  - pressing an arrow key on the keyboard

Additionally, because page layouts usually align things vertically and/or horizontally, UAs sometimes axis-lock a scroll when its direction is sufficiently vertical or horizontal. An axis-locked scroll is bound to only scroll along that axis. This prevents, for example, a nearly horizontal fling gesture from gradually drifting up or down as well, because it is very difficult to fling in a precisely horizontal line.

### 7.2. Axis vs Point-Snapping

This feature is planned to be removed in the next publication in order to reduce the feature-set of Level 1. It is included here for future reference in defining Level 2.

There are two distinct snapping behaviors that a scroll container might engage in:

axis-snapping

  If a scroll container is axis-snapping, its descendants indicate a desired scroll position in each axis of the scroll container independently, with no dependent preference for what the other axis’s scroll position should be.

  Note: This is the “default” type of snap behavior that most scroll containers will want to use, and so the scroll-snap-type property intentionally defaults to it.

  Note: An element in an axis-snapping scroll container can declare two snap positions, one in each axis. If one of the element’s snap positions is chosen in one axis, this has no bearing on the other dimension’s snap position—it might be chosen, or a different element’s snap position might be chosen for that axis, or that axis might not snap at all.

point-snapping

  If a scroll container is point-snapping, its descendants indicate a desired scroll position in both axises of the scroll container simultaneously—in other words, some point in the descendant must be aligned to a corresponding point in the scroll container.

  This type of snapping behavior is intended for "two-dimensional" panning-type layouts, such as cities on a map (using proximity 2D snap positions to snap a city to the center of the display when it gets close), or a tiled image gallery (using mandatory 2D snap positions to force each image to be centered on the screen). In both of these cases, it would look weird if the horizontal scrolling was aligned to one element while the vertical was aligned to a different element (which is the behavior you’d get if the scroll container was axis-snapping).

### 7.3. Choosing Snap Positions

A scroll container can have many snap areas scattered throughout its scrollable area. A naive algorithm for selecting a snap position can produce behavior that is unintuitive for users, so care is required when designing a selection algorithm. Here are a few pointers that can aid in the selection process:

- Snap positions should be chosen to minimize the distance between the end-point (or the natural end-point) and the final snapped scroll position, subject to the additional constraints listed in this section.

- Point-snapping is all-or-nothing; if the snap position of an element is chosen to align to, the scroll container must set its scroll position according to the element’s snap positions in both axises; the scroll container must not “partially align” to the element by taking its snap position in one axis and aligning the other axis according to something else.

- If a scroll is axis-locked and the scroll container is axis-snapping, any snap positions in the other axis should be ignored during the scroll. (However, snap positions in the other axis can still effect the final scroll position.)

  If a scroll is axis-locked and the scroll container is point-snapping, snap positions should be penalized in the selection process according to the amount of other-axis scrolling they would cause.

- Snap positions should be ignored if their elements are far outside of the “corridor” that the snapport defines as it moves through the scrollable area during an inertial scroll, or a hypothetical “corridor” in the direction of a directional scroll, or the snapport after an explicit scroll. (This is to prevent a far-offscreen element from having difficult-to-understand effects on the scroll position.)

- User agents must ensure that a user can “escape” a snap position, regardless of the scroll method. For example, if the snap type is mandatory and the next snap position is more than two screen-widths away, a naïve “always snap to nearest” selection algorithm would “trap” the user if they were panning with a touch gesture; a sufficiently large distance would even trap fling scrolling! Instead, a smarter algorithm that only returned to the starting snap position if the end-point was a fairly small distance from it, and otherwise ignored the starting snap position, would give better behavior.

  (This implies that a directional scroll must always ignore the starting snap positions.)

- If a page is navigated to a fragment that defines a target element (one that would be matched by :target), and that element defines some snap positions, the user agent should snap to one of that element’s snap positions. The user agent may do this even when the scroll container has scroll-snap-type: none.

## Appendix A: Longhands

### Physical Longhands for scroll-snap-padding

Name: | scroll-snap-padding-top, scroll-snap-padding-right, scroll-snap-padding-bottom, scroll-snap-padding-left
Value: | <length> | <percentage>
Initial: | 0
Applies to: | scroll containers
Inherited: | no
Percentages: | n/a
Computed value: | as specified, with lengths made absolute
Percentage: | relative to the corresponding dimension of the scroll container’s scrollport
Animatable: | as length, percentage, or calc

These longhands of scroll-snap-padding specify the top, right, bottom, and left edges of the snapport, respectively.

### Flow-relative Longhands for scroll-snap-padding

Name: | scroll-snap-padding-inline-start, scroll-snap-padding-block-start, scroll-snap-padding-inline-end, scroll-padding-block-end
Value: | <length> | <percentage>
Initial: | 0
Applies to: | scroll containers
Inherited: | no
Percentages: | relative to the corresponding dimension of the scroll container’s scrollport
Computed value: | as specified, with lengths made absolute
Animatable: | as length, percentage, or calc

These longhands of scroll-snap-padding specify the block-start, inline-start, block-end, and inline-end edges of the snapport, respectively.

Name: | scroll-snap-padding-block, scroll-snap-padding-inline
Value: | [ <length> | <percentage> ]{1,2}
Initial: | 0
Applies to: | all elements
Inherited: | no
Percentages: | relative to the corresponding dimension of the scroll container’s scrollport
Computed value: | as specified, with lengths made absolute
Animatable: | as length, percentage, or calc

These shorthands of scroll-snap-padding-block-start + scroll-snap-padding-block-end and scroll-snap-padding-inline-start + scroll-snap-padding-inline-end are longhands of scroll-snap-padding, and specify the block-axis and inline-axis edges of the snapport, respectively.

If two values are specified, the first gives the start value and the second gives the end value.

### Physical Longhands for scroll-snap-margin

Name: | scroll-snap-margin-top, scroll-snap-margin-right, scroll-snap-margin-bottom, scroll-snap-margin-left
Value: | <length>
Initial: | 0
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | as specified, with lengths made absolute
Animatable: | as length

These longhands of scroll-snap-margin specify the top, right, bottom, and left edges of the scroll snap area, respectively.

### Flow-relative Longhands for scroll-snap-margin

Name: | scroll-snap-margin-block-start, scroll-snap-margin-inline-start, scroll-snap-margin-block-end, scroll-snap-margin-inline-end
Value: | <length>
Initial: | 0
Applies to: | all elements
Inherited: | no
Percentages: | n/a
Computed value: | as specified, with lengths made absolute
Animatable: | as length

These longhands of scroll-snap-margin specify the block-start, inline-start, block-end, and inline-end edges of the scroll snap area, respectively.

Name: | scroll-snap-margin-block, scroll-snap-margin-inline
Value: | <length>{1,2}
Initial: | 0
Applies to: | all elements
Inherited: | no
Percentages: | n/a | Computed value: | as specified, with lengths made absolute
Animatable: | as length

These shorthands of scroll-snap-margin-block-start + scroll-snap-margin-block-end and scroll-snap-margin-inline-start + scroll-snap-margin-inline-end are longhands of scroll-snap-margin, and specify the block-axis and inline-axis edges of the scroll snap area, respectively. If two values are specified, the first gives the start value and the second gives the end value.

## 8. Privacy and Security Considerations

This specification does not expose any information whatsoever that is not already exposed to the DOM directly; it just makes scrolling slightly more functional. There are no new privacy or security considerations.

## 9. Acknowledgements

Many thanks to David Baron, Simon Fraser, Håkon Wium Lie, Edward O’Connor, François Remy, Majid Valpour, potentially some anonymous Microsoft engineers (?), and most especially Robert O’Callahan for their proposals and recommendations, which have been incorporated into this document.

## Conformance

### Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with `class="example"`, like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with `class="note"`, like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with `<strong class="advisement">`, like this: UAs MUST provide an accessible alternative.

### Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
  A CSS style sheet.
renderer
  A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
  A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

### Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

#### Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

#### Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

#### Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

## Index

### Terms defined by this specification

- always, in §6.3
- axis, in §7.2
- axis-lock, in §7.1
- axis-snap, in §7.2
- axis-snapping, in §7.2
- axis value, in §5.1
- block, in §5.1
- both, in §5.1
- captures snap positions, in §5.1
- center, in §6.2
- directional, in §7.1
- directional scroll, in §7.1
- end, in §6.2
- explicit, in §7.1
- explicit scroll, in §7.1
- inertial, in §7.1
- inertial scroll, in §7.1
- inline, in §5.1
- mandatory, in §5.1
- natural end-point, in §7.1
- none
  - value for scroll-snap-type, in §5.1
  - value for scroll-snap-align, in §6.2
- normal, in §6.3
- point
  - value for scroll-snap-type, in §5.1
  - definition of, in §7.2
- point-snap, in §7.2
- point-snapping, in §7.2
- proximity, in §5.1
- scroll-padding-block-end, in §Unnumbered section
- scroll-snap-align, in §6.2
- scroll snap area, in §6.1
- scroll snap container, in §5.1
- scroll-snap-margin, in §6.1
- scroll-snap-margin-block, in §Unnumbered section
- scroll-snap-margin-block-end, in §Unnumbered section
- scroll-snap-margin-block-start, in §Unnumbered section
- scroll-snap-margin-bottom, in §Unnumbered section
- scroll-snap-margin-inline, in §Unnumbered section
- scroll-snap-margin-inline-end, in §Unnumbered section
- scroll-snap-margin-inline-start, in §Unnumbered section
- scroll-snap-margin-left, in §Unnumbered section
- scroll-snap-margin-right, in §Unnumbered section
- scroll-snap-margin-top, in §Unnumbered section
- scroll-snap-padding, in §5.2
- scroll-snap-padding-block, in §Unnumbered section
- scroll-snap-padding-block-start, in §Unnumbered section
- scroll-snap-padding-bottom, in §Unnumbered section
- scroll-snap-padding-inline, in §Unnumbered section
- scroll-snap-padding-inline-end, in §Unnumbered section
- scroll-snap-padding-inline-start, in §Unnumbered section
- scroll-snap-padding-left, in §Unnumbered section
- scroll-snap-padding-right, in §Unnumbered section
- scroll-snap-padding-top, in §Unnumbered section
- scroll snapport, in §5.2
- scroll snap position, in §4
- scroll-snap-stop, in §6.3
- scroll-snap-type, in §5.1
- snap, in §4
- snap area, in §6.1
- snap behavior, in §7.2
- snap container, in §5.1
- snapped, in §4
- snapping, in §4
- snapping behavior, in §7.2
- snapport, in §5.2
- snap position, in §4
- start, in §6.2
- strictness value, in §5.1
- x, in §5.1
- y, in §5.1

### Terms defined by reference

- [css-align-3] defines the following terms:
  - align-self
  - alignment container
  - alignment subject
  - justify-self
- [css-backgrounds-3] defines the following terms:
  - <position>
  - border-image-outset
- [css-cascade-4] defines the following terms:
  - longhand
  - shorthand
  - shorthand property
- [css-display-3] defines the following terms:
  - block
  - containing block chain
  - inline
- [css-overflow-3] defines the following terms:
  - scroll container
- [css-overflow-4] defines the following terms:
  - auto
  - overflow
  - scroll
- [css-values] defines the following terms:
  - <length>
  - <percentage>
  - {a,b}
  - |
  - ||
- [css-writing-modes-3] defines the following terms:
  - block axis
  - inline axis
  - writing mode
- [CSS2] defines the following terms:
  - margin
  - padding
- [cssom-view-1] defines the following terms:
  - scrollBy()
  - scrollTo()
- [selectors-4] defines the following terms:
  - :target

## References

### Normative References

[CSS-ALIGN-3]
  Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 14 June 2016. WD. URL: https://drafts.csswg.org/css-align/
[CSS-BACKGROUNDS-3]
  CSS Backgrounds and Borders Module Level 3 URL: https://drafts.csswg.org/css-backgrounds-3/
[CSS-CASCADE-4]
  Elika Etemad; Tab Atkins Jr.. CSS Cascading and Inheritance Level 4. 14 January 2016. CR. URL: http://dev.w3.org/csswg/css-cascade/
[CSS-DISPLAY-3]
  Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 15 October 2015. WD. URL: http://dev.w3.org/csswg/css-display/
[CSS-OVERFLOW-3]
  David Baron; Florian Rivoal. CSS Overflow Module Level 3. 31 May 2016. WD. URL: http://drafts.csswg.org/css-overflow/
[CSS-OVERFLOW-4]
  CSS Overflow Module Level 4 URL: https://drafts.csswg.org/css-overflow-4/
[CSS-VALUES]
  Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 11 June 2015. CR. URL: http://dev.w3.org/csswg/css-values/
[CSS-WRITING-MODES-3]
  Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 15 December 2015. CR. URL: http://dev.w3.org/csswg/css-writing-modes-3/
[CSS2]
  Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS2
[CSSOM-VIEW-1]
  Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://drafts.csswg.org/cssom-view/
[RFC2119]
  S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SELECTORS-4]
  Selectors Level 4 URL: https://drafts.csswg.org/selectors-4/

## Property Index

Name | Value | Initial | Applies to | Inh. | %ages | Media | Animatable | Computed value | Percentage
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
scroll-snap-type | none | [ proximity | mandatory ] || [ x | y | block | inline | both | point ] | none | all elements | no | n/a | interactive | no | as specified |
scroll-snap-padding | [ <length> | <percentage> ]{1,4} | 0 | scroll containers | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-margin | <length>{1,4} | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-align | [ none | start | end | center ]{1,2} | none | all elements | no | n/a | interactive | no | two keywords |
scroll-snap-stop | normal | always | normal | all elements | no | n/a | interactive | no | as specified |
scroll-snap-padding-top | <length> | <percentage> | 0 | scroll containers | no | n/a | interactive | as length, percentage, or calc | as specified, with lengths made absolute | relative to the corresponding dimension of the scroll container’s scrollport
scroll-snap-padding-right | <length> | <percentage> | 0 | scroll containers | no | n/a | interactive | as length, percentage, or calc | as specified, with lengths made absolute | relative to the corresponding dimension of the scroll container’s scrollport
scroll-snap-padding-bottom | <length> | <percentage> | 0 | scroll containers | no | n/a | interactive | as length, percentage, or calc | as specified, with lengths made absolute | relative to the corresponding dimension of the scroll container’s scrollport
scroll-snap-padding-left | <length> | <percentage> | 0 | scroll containers | no | n/a | interactive | as length, percentage, or calc | as specified, with lengths made absolute | relative to the corresponding dimension of the scroll container’s scrollport
scroll-snap-padding-inline-start | <length> | <percentage> | 0 | scroll containers | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-padding-block-start | <length> | <percentage> | 0 | scroll containers | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-padding-inline-end | <length> | <percentage> | 0 | scroll containers | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-padding-block-end | <length> | <percentage> | 0 | scroll containers | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-padding-block | [ <length> | <percentage> ]{1,2} | 0 | all elements | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-padding-inline | [ <length> | <percentage> ]{1,2} | 0 | all elements | no | relative to the corresponding dimension of the scroll container’s scrollport | interactive | as length, percentage, or calc | as specified, with lengths made absolute |
scroll-snap-margin-top | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-right | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-bottom | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-left | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-block-start | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-inline-start | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-block-end | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-inline-end | <length> | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-block | <length>{1,2} | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |
scroll-snap-margin-inline | <length>{1,2} | 0 | all elements | no | n/a | interactive | as length | as specified, with lengths made absolute |

## Issues Index

We’re considering splitting this into subproperties. Current proposed names are scroll-snap for the current grammar, scroll-snap-affinity for the proximity/mandatory distinction, and scroll-snap-axis for the x/y/etc distinction. ↵
Is this the correct writing mode to compute against? Or should it be the scroll container’s writing mode? ↵
This feature is planned to be removed in the next publication in order to reduce the feature-set of Level 1. It is included here for future reference in defining Level 2. ↵
#scroll-snap-position Referenced in:

- 4. Scroll Snapping Model (2) (3) (4) (5) (6) (7)
- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
- 5.2. Scroll Snapport: the scroll-snap-padding property
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property (2) (3) (4) (5) (6) (7)
- 6.2.1. Scoping Valid Snap Positions to Visible Boxes (2)
- 6.2.2. Snapping Boxes that Overflow the Scrollport (2) (3)
- 6.2.3. Unreachable Snap Areas (2) (3)
- 6.3. Scroll Snap Limits: the scroll-snap-stop property (2) (3) (4) (5) (6) (7) (8)
- 7. Snapping Mechanics
- 7.2. Axis vs Point-Snapping (2) (3) (4)
- 7.3. Choosing Snap Positions (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

#snap Referenced in:

- 4. Scroll Snapping Model
- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2) (3) (4) (5) (6) (7) (8) (9) (10)
- 7.3. Choosing Snap Positions

#propdef-scroll-snap-type Referenced in:

- 3. Overview of Change (2)
- 4. Scroll Snapping Model
- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2) (3) (4)
- 7.3. Choosing Snap Positions

#valdef-scroll-snap-type-none Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2) (3)

#valdef-scroll-snap-type-proximity Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2)
- 7.2. Axis vs Point-Snapping

#valdef-scroll-snap-type-mandatory Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property
- 7.2. Axis vs Point-Snapping
- 7.3. Choosing Snap Positions

#captures-snap-positions Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property

#scroll-snap-container Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2)
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property

#propdef-scroll-snap-padding Referenced in:

- 4. Scroll Snapping Model
- 5.2. Scroll Snapport: the scroll-snap-padding property (2)
- Physical Longhands for scroll-snap-padding (2)
- Flow-relative Longhands for scroll-snap-padding (2) (3)

#scroll-snapport Referenced in:

- 4. Scroll Snapping Model
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property (2) (3) (4) (5) (6) (7)
- 6.2.1. Scoping Valid Snap Positions to Visible Boxes (2) (3) (4)
- 6.2.2. Snapping Boxes that Overflow the Scrollport (2) (3) (4) (5)
- 7.3. Choosing Snap Positions (2)
- Physical Longhands for scroll-snap-padding
- Flow-relative Longhands for scroll-snap-padding (2)

#propdef-scroll-snap-margin Referenced in:

- 4. Scroll Snapping Model
- 6.1. Scroll Snapping Margin: the scroll-snap-margin property (2)
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property (2)
- Physical Longhands for scroll-snap-margin (2)
- Flow-relative Longhands for scroll-snap-margin (2) (3)

#scroll-snap-area Referenced in:

- 4. Scroll Snapping Model
- 5.2. Scroll Snapport: the scroll-snap-padding property
- 6.1. Scroll Snapping Margin: the scroll-snap-margin property (2)
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property (2) (3) (4) (5)
- 6.2.1. Scoping Valid Snap Positions to Visible Boxes (2) (3)
- 6.2.2. Snapping Boxes that Overflow the Scrollport (2) (3) (4) (5)
- 6.2.3. Unreachable Snap Areas
- 7.3. Choosing Snap Positions
- Physical Longhands for scroll-snap-margin
- Flow-relative Longhands for scroll-snap-margin (2)

#propdef-scroll-snap-align Referenced in:

- 3. Overview of Change
- 4. Scroll Snapping Model
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property (2)

#valdef-scroll-snap-align-none Referenced in:

- 6.2. Scroll Snapping Alignment: the scroll-snap-align property

#propdef-scroll-snap-stop Referenced in:

- 6.3. Scroll Snap Limits: the scroll-snap-stop property (2)

#explicit-scroll Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property
- 7.1. Types of Scrolling Methods
- 7.3. Choosing Snap Positions

#inertial-scroll Referenced in:

- 6.3. Scroll Snap Limits: the scroll-snap-stop property (2) (3) (4) (5)
- 7.1. Types of Scrolling Methods (2)
- 7.3. Choosing Snap Positions

#natural-end-point Referenced in:

- 7.3. Choosing Snap Positions

#directional-scroll Referenced in:

- 7.1. Types of Scrolling Methods
- 7.3. Choosing Snap Positions (2)

#axis-lock Referenced in:

- 7.1. Types of Scrolling Methods
- 7.3. Choosing Snap Positions (2)

#snap-behavior Referenced in:

- 7.2. Axis vs Point-Snapping (2)

#axis-snapping Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property (2) (3) (4) (5) (6) (7)
- 7.2. Axis vs Point-Snapping (2) (3)
- 7.3. Choosing Snap Positions

#point-snapping Referenced in:

- 5.1. Scroll Snapping Rules: the scroll-snap-type property
- 6.2. Scroll Snapping Alignment: the scroll-snap-align property
- 7.2. Axis vs Point-Snapping
- 7.3. Choosing Snap Positions (2)

#propdef-scroll-snap-padding-inline-start Referenced in:

- Flow-relative Longhands for scroll-snap-padding

#propdef-scroll-snap-padding-block-start Referenced in:

- Flow-relative Longhands for scroll-snap-padding

#propdef-scroll-snap-padding-inline-end Referenced in:

- Flow-relative Longhands for scroll-snap-padding

#propdef-scroll-snap-margin-block-start Referenced in:

- Flow-relative Longhands for scroll-snap-margin

#propdef-scroll-snap-margin-inline-start Referenced in:

- Flow-relative Longhands for scroll-snap-margin

#propdef-scroll-snap-margin-block-end Referenced in:

- Flow-relative Longhands for scroll-snap-margin

#propdef-scroll-snap-margin-inline-end Referenced in:

- Flow-relative Longhands for scroll-snap-margin