# CSS TV Profile 1.0

## W3C Working Group Note 10 October 2014

This version:
  http://www.w3.org/TR/2014/NOTE-css-tv-20141010/
Latest version:
  http://www.w3.org/TR/css-tv
Previous version:
  http://www.w3.org/TR/2003/CR-css-tv-20030514
Authors:
  Sean Hayes (Microsoft) <shayes@microsoft.com>
  Glenn Adams (Cox Communications, Inc.) <glenn@skynav.com>
  Tantek Çelik (Mozilla Foundation) <tantek@cs.stanford.edu>
  Håkon Wium Lie (Opera Software) <howcome@opera.com>

Copyright © 2014 W3C® (MIT, ERCIM, Keio, Beihang), All Rights Reserved. W3C liability, trademark and document use rules apply.

## Abstract

This specification defines a subset of Cascading Style Sheets Level 2 and CSS3 module: Color specifications tailored to the needs and constraints of TV devices.

This profile is obsolete. Please see the latest CSS Snapshot for the specifications that make up CSS.

## Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by the CSS Working Group (part of the Style Activity).

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this document. When sending e-mail, please put the text “css-tv” in the subject, preferably like this: “[css-tv] …summary of comment…”

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 August 2014 W3C Process Document.

At this time, the CSS Working Group does not envisage further work on this specification and does not plan to propose it as a W3C Recommendation.

## 1. Overview

This document specifies a profile of the Cascading Style Sheets level 2 (CSS2) and CSS3 module: Color specifications appropriate for TV devices such as set top boxes or integrated interactive television sets that display their output on a television screen. Conformance to this profile means that a user agent supports, at minimum, the features defined in this specification. This subject is addressed in Section 2, Conformance, below.

As defined in [CSS2]:

> CSS2 is a style sheet language that allows authors and users to attach style (e.g., fonts, spacing, and aural cues) to structured documents (e.g., HTML documents and XML applications). By separating the presentation style of documents from the content of documents, CSS2 simplifies Web authoring and site maintenance. CSS2 builds on CSS1 (see [CSS1]) and, with very few exceptions, all valid CSS1 style sheets are valid CSS2 style sheets. CSS2 supports media-specific style sheets so that authors may tailor the presentation of their documents to visual browsers, aural devices, printers, braille devices, handheld devices, etc.

In summary, CSS2 specifies how developers can author style sheets for presenting documents across multiple devices and media types. While this is very important, it is also important that authors have an understanding of what features are supported on these different devices. Likewise, it is important that similar devices operate in a similar manner. Otherwise, authors will need to develop style sheets for each version of each device -- raising the cost of content development and decreasing interoperability.

The CSS TV Profile specifies a conformance profile for TV devices, identifying a minimum set of properties, values, selectors, and cascading rules. The resulting CSS TV Profile includes the vast majority of CSS1, portions of CSS2 and CSS3 module: Color. The CSS TV Profile is a proper superset of the CSS Mobile Profile, with the use of the ‘`tv`’ media type instead of the ‘`handheld`’ media type.

## 2. Conformance

The primary role of a profile is to define a subset of features that provides a minimal guarantee of interoperability. In the case of the CSS TV Profile, this guarantee is that a conforming user agent will support the features defined in this specification following the CSS2 conformance clause ([CSS2] Section 3.2), recast and summarized below:

1. A CSS TV Profile conforming user agent (TV-UA) MUST support the all and tv CSS2 media types. A TV-UA MAY support other CSS2 media types, as well.
2. For each source document, a TV-UA MUST attempt to retrieve all associated style sheets that are appropriate for the supported media types.
3. A TV-UA MUST parse the style sheets according to this specification. In particular, the TV-UA MUST recognize all CSS TV Profile at-rules, blocks, declarations, and selectors. If a TV-UA encounters a property that applies for a supported media type, the TV-UA MUST parse the value according to the property definition. This means that the TV-UA MUST accept all valid values and MUST ignore declarations with invalid values. TV-UA MUST ignore rules that apply to unsupported media types.
4. For each element in a document tree, the TV-UA MUST assign a value for every applicable property according to the property's definition and the rules of cascading and inheritance.
5. If the source document comes with alternate style sheets (such as with the "alternate" keyword in HTML 4.0 [HTML40]), the TV-UA SHOULD allow the user to select one from among these style sheets and apply the selected one.

As with CSS2, there are qualifications to this conformance clause:

1. Values MAY be approximated when required by the TV-UA.
2. The inability of a TV-UA to implement part of this specification due to the limitations of a particular device (e.g., a TV-UA cannot render colors on a monochrome monitor or page) SHALL NOT imply non-conformance.

It is recommended that authors use this conformance profile to take advantage of forward compatibility. Authors should be able to use style properties with an understanding that the cascading rules are processed correctly and that unknown properties and values are ignored. For example:

      h3 {
        display: inline;
        display: run-in;
      }

A TV-UA that can process the ‘`run-in`’ value for the ‘`display`’ property will accept the first display declaration and then "write over" that value with the second display declaration. A TV-UA that cannot process the ‘`run-in`’ value will process the first display specification and ignore the second display specification.

## 3. Selectors and at-rules

### 3.1. Selectors

In CSS2, pattern matching rules determine which style rules apply to elements in the document tree [CSS2].

The following table summarizes CSS TV Profile selector syntax. In addition to these selectors, the CSS TV Profile includes the CSS2 grouping mechanism (See [CSS2] Section 5.2.1).

Note. Unsupported selectors are parsed as invalid (which would affect valid selectors in the same group), the same as if a purely CSS1 UA were to encounter selectors new in CSS2. Of course a TV-UA is free to implement more selectors than required by CSS TV.

Pattern | Meaning | Selector type | CSS TV
--- | --- | --- | ---
* | Matches any element | Universal selector | Yes
E | Matches any E element (i.e., and element of type E) | Type selectors | Yes
E F | Matches any F element that is a descendent of an E element | Descendent selectors | Yes
E > F | Matches any F element that is a child of an element E | Child selectors | Yes
E:first-child | Matches element E when it is the first child of its parent | The :first-child pseudo-class | Yes
E:link E:visited | Matches element E if E is the source anchor of a hyperlink of which the target is not yet visited (:link) or already visited (:visited). | The link pseudo-classes | Yes
E:active | Matches E during certain user actions. | The dynamic pseudo-classes | Yes
E:hover | Matches E during certain user actions. | The dynamic pseudo-classes | No
E:focus | Matches E during certain user actions. | The dynamic pseudo-classes | Yes
E:lang(c) | Matches element of type E if it is in (human) language c (the document language specifies how language is determined). | The :lang() pseudo-class | No
E + F | Matches any F element immediately preceded by an element E. | Adjacent selectors | No
E[foo] | Matches any E element with the "foo" attribute set (whatever the value). | Attribute selectors | No
E[foo="bar"] | Matches any E element whose "foo" attribute value is exactly equal to "bar". | Attribute selectors | No
E[foo~="bar"] | Matches any E element whose "foo" attribute value is a list of space-separated values, one of which is exactly equal to "bar". | Attribute selectors | No
E[foo|="bar"] | Matches any E element whose "foo" attribute value has a hyphen-separated list of values beginning (from the left) with "bar". | Attribute selectors | No
E:first-line | Matches the first formatted line of an E element. | The :first-line pseudo-element | Yes
E:first-letter | Matches the first formatted letter of an E element. | The :first-letter pseudo-element | Yes
E:before | Matches/creates generated content before an E element. | The :before pseudo-element | No
E:after | Matches/creates generated content after an E element. | The :after pseudo-element | No
E.bar | Matches any E element whose class attribute (as defined by the document language) value is a list of space-separated values, one of which is exactly equal to "bar". | Class selectors | Yes
E#bar | Matches any E element whose ID attribute (as defined by the document language) is equal to "bar". | ID selectors | Yes

### 3.2. at-rules

The following table summarizes CSS TV Profile at-rule syntax.

at-rule | Function | CSS TV
--- | --- | ---
@import | Imports an external style sheet. | Yes
@charset | Defines character set for the style sheet. | Yes
@media | Groups a set of style rules to apply only to one or more particular media. | Yes
@font-face | Defines a named font-family, including for downloading. | No
@page | Defines a (optionally named) page formatting context. | No
@color-profile | Defines a named color-profile. | No

## 4. Properties

The following table summarizes CSS TV Profile properties and property values from CSS2. Refer to [CSS2] for the definition of these properties and values.

Name | CSS TV | CSS Values | Initial value
--- | --- | --- | ---
‘`azimuth`’ | No | <angle> | [ left-side | far-left | left | center-left | center | center-right | right | far-right | right-side ] || behind ] | leftwards | rightwards | inherit | center
‘`background`’ | [‘`background-color`’ || ‘`background-image`’ || ‘`background-repeat`’ || ‘`background-position`’] | inherit | [‘`background-color`’ || ‘`background-image`’ || ‘`background-repeat`’ || ‘`background-attachment`’ || ‘`background-position`’] | inherit | see individual properties
‘`background-attachment`’ | No | scroll | fixed | inherit | scroll
‘`background-color`’ | Yes | <color> | transparent | inherit | transparent
‘`background-image`’ | Yes | <uri> | none | inherit | none
‘`background-position`’ | Yes | [ [ <percentage> | <length> ]{1,2} | [ [top | center | bottom] || [left | center | right] ] ] | inherit | 0% 0%
‘`background-repeat`’ | Yes | repeat | repeat-x | repeat-y | no-repeat | inherit | repeat
‘`border`’ | Yes | [ <border-width> || <border-style> || <border-color> ] | inherit | see individual properties
‘`border-collapse`’ | No | collapse | separate | inherit | separate
‘`border-color`’ | Yes | <color>{1,4} | transparent | inherit | see individual properties
‘`border-spacing`’ | No | <length> <length>? | inherit | 0
‘`border-style`’ | Yes | <border-style>{1,4} | inherit | see individual properties
‘`border-top`’ ‘`border-right`’ ‘`border-bottom`’ ‘`border-left`’ | Yes | [ <border-width> || <border-style> || <border-color> ] | inherit | see individual properties
‘`border-top-color`’ ‘`border-right-color`’ ‘`border-bottom-color`’ ‘`border-left-color`’ | Yes | <color> | transparent | inherit | the value of the ‘`color`’ property
‘`border-top-style`’ ‘`border-right-style`’ ‘`border-bottom-style`’ ‘`border-left-style`’ | Yes | <border-style> | inherit | none
‘`border-top-width`’ ‘`border-right-width`’ ‘`border-bottom-width`’ ‘`border-left-width`’ | Yes | <border-width> | inherit | medium
‘`border-width`’ | Yes | <border-width>{1,4} | inherit | see individual properties
‘`bottom`’ | Yes | <length> | <percentage> | auto | inherit | auto
‘`caption-side`’ | Yes | top | bottom | left | right | inherit | top
‘`clear`’ | Yes | none | left | right | both | inherit | none
‘`clip`’ | Yes | <shape> | auto | inherit | auto
‘`color`’ | Yes | <color> | inherit | depends on user agent
‘`content`’ | No | [ <string> | <uri> | <counter> | attr(X) | open-quote | close-quote | no-open-quote | no-close-quote ]+ | inherit | empty string
‘`counter-increment`’ | No | [ <identifier> <integer>? ]+ | none | inherit | none
‘`counter-reset`’ | No | [ <identifier> <integer>? ]+ | none | inherit | none
‘`cue`’ | No | [ ‘`cue-before`’ || ‘`cue-after`’ ] | inherit | see individual properties
‘`cue-after`’ | No | <uri> | none | inherit | none
‘`cue-before`’ | No | <uri> | none | inherit | none
‘`cursor`’ | No | [ [<uri> ,]* [ auto | crosshair | default | pointer | move | e-resize | ne-resize | nw-resize | n-resize | se-resize | sw-resize | s-resize | w-resize| text | wait | help ] ] | inherit | auto
‘`direction`’ | No | ltr | rtl | inherit | ltr
‘`display`’ | inline | block | list-item | none | inherit | inline | block | list-item | run-in | compact | marker | table | inline-table | table-row-group | table-header-group | table-footer-group | table-row | table-column-group | table-column | table-cell | table-caption | none | inherit | inline
‘`elevation`’ | No | <angle> | below | level | above | higher | lower | inherit | level
‘`empty-cells`’ | No | show | hide | inherit | show
‘`float`’ | Yes | left | right | none | inherit | none
‘`font`’ | Yes | [ [ ‘`font-style`’ || ‘`font-variant`’ || ‘`font-weight`’ ]? ‘`font-size`’ [ / ‘`line-height`’ ]? ‘`font-family`’ ] | caption | icon | menu | message-box | small-caption | status-bar | inherit | see individual properties
‘`font-family`’ | Yes | [[ <family-name> | <generic-family> ],]* [ <family-name> | <generic-family> ] | inherit | depends on user agent
‘`font-size`’ | Yes | <absolute-size> | <relative-size> | <length> | <percentage> | inherit | medium
‘`font-size-adjust`’ | No | <number> | none | inherit | none
‘`font-stretch`’ | No | normal | wider | narrower | ultra-condensed | extra-condensed | condensed | semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded | inherit | normal
‘`font-style`’ | Yes | normal | italic | oblique | inherit | normal
‘`font-variant`’ | Yes | normal | small-caps | inherit | normal
‘`font-weight`’ | Yes | normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | inherit | normal
‘`height`’ | Yes | <length> | <percentage> | auto | inherit | auto
‘`left`’ | Yes | <length> | <percentage> | auto | inherit | auto
‘`letter-spacing`’ | No | normal | <length> | inherit | normal
‘`line-height`’ | Yes | normal | <number> | <length> | <percentage> | inherit | normal
‘`list-style`’ | Yes | [ ‘`list-style-type`’ || ‘`list-style-position`’ || ‘`list-style-image`’ ] | inherit | see individual properties
‘`list-style-image`’ | Yes | <uri> | none | inherit | none
‘`list-style-position`’ | Yes | inside | outside | inherit | outside
‘`list-style-type`’ | disc | circle | square | decimal | lower-roman | upper-roman | lower-alpha | lower-latin | upper-alpha | upper-latin | none | inherit | disc | circle | square | decimal | decimal-leading-zero | lower-roman | upper-roman | lower-greek | lower-alpha | lower-latin | upper-alpha | upper-latin | hebrew | armenian | georgian | cjk-ideographic | hiragana | katakana | hiragana-iroha | katakana-iroha | none | inherit | disc
‘`margin`’ | Yes | <margin-width>{1,4} | inherit | see individual properties
‘`margin-top`’ ‘`margin-right`’ ‘`margin-bottom`’ ‘`margin-left`’ | Yes | <margin-width> | inherit | 0
‘`marker-offset`’ | No | <length> | auto | inherit | auto
‘`marks`’ | No | [ crop || cross ] | none | inherit | none
‘`max-height`’ | No | <length> | <percentage> | none | inherit | none
‘`max-width`’ | No | <length> | <percentage> | none | inherit | none
‘`min-height`’ | No | <length> | <percentage> | inherit | 0
‘`min-width`’ | No | <length> | <percentage> | inherit | depends on user agent
‘`orphans`’ | No | <integer> | inherit | 2
‘`outline`’ | Yes | [ ‘`outline-color`’ || ‘`outline-style`’ || ‘`outline-width`’ ] | inherit | see individual properties
‘`outline-color`’ | Yes | <color> | invert | inherit | invert
‘`outline-style`’ | Yes | <border-style> | inherit | none
‘`outline-width`’ | Yes | <border-width> | inherit | medium
‘`overflow`’ | No | visible | hidden | scroll | auto | inherit | visible
‘`padding`’ | Yes | <padding-width>{1,4} | inherit | see individual properties
‘`padding-top`’ ‘`padding-right`’ ‘`padding-bottom`’ ‘`padding-left`’ | Yes | <padding-width> | inherit | 0
‘`page`’ | No | <identifier> | auto | auto
‘`page-break-after`’ | No | auto | always | avoid | left | right | inherit | auto
‘`page-break-before`’ | No | auto | always | avoid | left | right | inherit | auto
‘`page-break-inside`’ | No | avoid | auto | inherit | auto
‘`pause`’ | No | [ [<time> | <percentage>]{1,2} ] | inherit | depends on user agent
‘`pause-after`’ | No | <time> | <percentage> | inherit | depends on user agent
‘`pause-before`’ | No | <time> | <percentage> | inherit | depends on user agent
‘`pitch`’ | No | <frequency> | x-low | low | medium | high | x-high | inherit | medium
‘`pitch-range`’ | No | <number> | inherit | 50
‘`play-during`’ | No | <uri> mix? repeat? | auto | none | inherit | auto
‘`position`’ | static | relative | absolute | inherit | static | relative | absolute | fixed | inherit | static
‘`quotes`’ | No | [ <string><string>]+ | none | inherit | depends on user agent
‘`richness`’ | No | <number> | inherit | 50
‘`right`’ | Yes | <length> | <percentage> | auto | inherit | auto
‘`size`’ | No | <length>{1,2} | auto | portrait | landscape | inherit | auto
‘`speak`’ | No | normal | none | spell-out | inherit | normal
‘`speak-header`’ | No | once | always | inherit | once
‘`speak-numeral`’ | No | digits | continuous | inherit | continuous
‘`speak-punctuation`’ | No | code | none | inherit | none
‘`speech-rate`’ | No | <number> | x-slow | slow | medium | fast | x-fast | faster | slower | inherit | medium
‘`stress`’ | No | <number> | inherit | 50
‘`table-layout`’ | No | auto | fixed | inherit | auto
‘`text-align`’ | left | right | center | justify | inherit | left | right | center | justify | <string> | inherit | depends on user agent and writing direction
‘`text-decoration`’ | none | [ underline || overline || line-through ] | inherit | none | [ underline || overline || line-through || blink ] | inherit | none
‘`text-indent`’ | Yes | <length> | <percentage> | inherit | 0
‘`text-shadow`’ | No | none | [<color> || <length> <length> <length>? ,]* [ <color> || <length> <length> <length>?] | inherit | none
‘`text-transform`’ | Yes | capitalize | uppercase | lowercase | none | inherit | none
‘`top`’ | Yes | <length> | <percentage> | auto | inherit | auto
‘`unicode-bidi`’ | No | normal | embed | bidi-override | inherit | normal
‘`vertical-align`’ | baseline | sub | super | top | middle | bottom | inherit | baseline | sub | super | top | text-top | middle | bottom | text-bottom | <percentage> | <length> | inherit | baseline
‘`visibility`’ | Yes | visible | hidden | collapse | inherit | inherit
‘`voice-family`’ | No | [[ <specific-voice> | <generic-voice> ],]* [ <specific-voice> | <generic-voice> ] | inherit | depends on user agent
‘`volume`’ | No | <number> | <percentage> | silent | x-soft | soft | medium | loud | x-loud | inherit | medium
‘`white-space`’ | Yes | normal | pre | nowrap | inherit | normal
‘`widows`’ | No | <integer> | inherit | 2
‘`width`’ | Yes. Note that a TV UA may implement a UA style sheet rule of `*{max-width: 100%}` to avoid horizontal scrolling. | <length> | <percentage> | auto | inherit | auto
‘`word-spacing`’ | No | normal | <length> | inherit | normal
‘`z-index`’ | Yes | auto | <integer> | inherit | auto

The following table summarizes CSS TV Profile properties and property values from the CSS3 module: Color. Refer to [CSS3COLOR] for the definition of these properties and values.

Name | CSS TV | CSS Values | Initial value
--- | --- | --- | ---
‘`color`’ | HTML4 keywords, RGB and RGBA values, transparent, orange, inherit | HTML4 keywords, RGB, RGBA, HSL, and HSLA values, transparent, X11 keywords, System Colors, flavor, attr(X,color), inherit | depends on user agent
‘`color-profile`’ | No | auto | sRGB | <name> | <uri> | inherit | auto
‘`opacity`’ | Yes | <alphavalue> <priority-index>? | inherit | 1 none
‘`rendering-intent`’ | no | auto | perceptual | relative-colorimetric | saturation | absolute-colorimetric | inherit | auto

## 5. CSS Syntax

The CSS TV Profile uses the same syntax as specified in [CSS2]. The CSS TV Profile uses a subset of the values used in CSS2. Specifically:

1. The TV-UA SHALL support integer and real numbers ([CSS2] Section 4.3.1).
2. The TV-UA SHALL support the following lengths ([CSS2] Section 4.3.2):
   - px
   - em
   - ex
   - in
   - cm
   - mm
   - pt
   - pc
   The TV-UA MAY support other lengths.
3. The TV-UA SHALL support percentage values ([CSS2] Section 4.3.3).
4. The TV-UA SHALL support URI values ([CSS2] Section 4.3.4).
5. The TV-UA MAY support counter values ([CSS2] Section 4.3.5).
6. The TV-UA SHALL support the following color values ([CSS2] Section 4.3.6):
   - The 16 named colors defined in HTML 4.0 [HTML401], orange and transparent as defined in CSS3 module: Color [CSS3COLOR]
   - A numerical RGB specification ([CSS2] Section 4.3.6)
   - A numerical RGBA specification as defined in [CSS3COLOR] Section 4.2.2
   The TV-UA MAY support other color values.

Similarly, the CSS TV Profile requires that conforming user agents support the character encoding mechanisms specified in [CSS2]. Specifically:

1. The TV-UA SHALL support priorities specified in [CSS2] to determine a document's character encoding.
2. The TV-UA SHALL support the CSS2 @charset rules.

## 6. Assigning Property Values, Cascading, and Inheritance

In general, the CSS TV Profile uses the same cascading rules as in CSS2. Specifically:

1. The TV-UA SHALL assign values as described in CSS2 ([CSS2] Section 6.1).
2. The TV-UA SHALL support inheritance as described in CSS2 ([CSS2] Section 6.2).
3. The TV-UA SHALL support the CSS2 @import rules as specified in CSS2 ([CSS2] Section 6.3).
4. The TV-UA SHALL support author originating style sheets. The TV-UA MAY support user or user-agent originating style sheets ([CSS2] Section 6.4).
5. The TV-UA SHALL support all CSS2 cascading mechanisms ([CSS2] Sections 6.4.1-6.4.4).

## 7. Media Types

A CSS TV Profile conforming user agent MUST be able to process media-dependent stylesheets as specified in CSS2 ([CSS2] Section 7). Specifically:

1. The TV-UA SHALL support the CSS2 @media rules as specified in CSS2 ([CSS2] Section 7).
2. The TV-UA SHALL accept and process style sheets that target the `tv` media type.
3. The TV-UA SHALL accept and process style sheets that target the `all` media type.
4. The TV-UA SHALL accept style sheets that contain other (non-tv) media-dependent style sheets.
5. The TV-UA MAY process other media types (such as `screen` or `print`).

The TV-UA is not required to satisfy the CSS2 conformance statement pertaining to the `tv` media type (see [CSS2] Section 7.3.1); the TV-UA need only satisfy the conformance statements in this specification.

## 8. Acknowledgements

This document derives from the CSS Level 1 and CSS level 2 Recommendations, the CSS3 module: Color and the CSS Mobile Profile 1.0. We thank all CSS1, CSS2, CSS3 module: Color and CSS Mobile Profile authors, editors and contributors.

We would like to thank the members of the CSS working group, especially Bert Bos, Chris Lilley, and Tapas Kanti Roy for their direct feedback on the draft.

We would like to thank the contributors on www-style@w3.org. We would like to especially thank ARIB, Akihiko Handa, Joel Zdepski, Etan Wexler, Kynn Bartlett, Björn Höhrmann, and fantasai for their valuable comments on the draft.

## Appendix A. Normative References

[CSS1]
  Håkon Wium Lie; Bert Bos. Cascading Style Sheets (CSS1) Level 1 Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS1-20080411
[CSS2]
  Ian Jacobs; et al. Cascading Style Sheets, level 2 (CSS2) Specification. 11 April 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-CSS2-20080411
[CSS3COLOR]
  Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607
[HTML401]
  Dave Raggett; Arnaud Le Hors; Ian Jacobs. HTML 4.01 Specification. 24 December 1999. W3C Recommendation. URL: http://www.w3.org/TR/1999/REC-html401-19991224
[RFC2119]
  S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt