CSS Generated Content Module Level 3

W3C Working Draft,

This version:
http://www.w3.org/TR/2016/WD-css-content-3-20160524/
Latest published version:
https://www.w3.org/TR/css3-content/
Editor's Draft:
https://drafts.csswg.org/css-content/
Previous Versions:
https://www.w3.org/TR/2003/WD-css3-content-20030514/
Feedback:
www-style@w3.org with subject line “[css-content] … message topic …” (archives)
Issue Tracking:
Inline In Spec
Editors:
Elika J. Etemad / fantasai (W3C Invited Expert)
(Hachette Livre)
Former Editors:
(Opera Software)
(Google)
Not Ready For Implementation

This spec is not yet ready for implementation. It exists in this repository to record the ideas and promote discussion.

Before attempting to implement this spec, please contact the CSSWG at www-style@w3.org.


Abstract

This CSS3 Module describes how to insert content in a document.

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

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 Draft 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.

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

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

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 September 2015 W3C Process Document.

Introduction

Authors sometimes want user agents to render content that does not come from the document tree. One familiar example of this is numbered headings; the author does not want to mark the numbers up explicitly, she or he wants the user agent to generate them automatically. Counters and markers are used to achieve these effects.

h1::before { content: counter(section) ": "; }

Similarly, authors may want the user agent to insert the word "Figure" before the caption of a figure, or "Chapter 7" on a line before the seventh chapter title.

chapter { counter-increment: chapter; }
chapter > title::before { content: "Chapter " counter(chapter) "\A"; }

Another common effect is replacing elements with images or other multimedia content. Since not all user agents support all multimedia formats, fallbacks may have to be provided.

/* Replace <logo> elements with the site’s logo, using a format
 * supported by the UA */
logo { content: url(logo.mov), url(logo.mng), url(logo.png), none; }

/* Replace <figure> elements with the referenced document, or,
 * failing that, with either the contents of the alt attribute or the
 * contents of the element itself if there is no alt attribute */
figure[alt] { content: attr(href, url), attr(alt); }
figure:not([alt]) { content: attr(href, url), contents; }

1. Inserting and replacing content with the content property

Name: content
Value: [ [ <image> | <url> ] ',' ]* [ normal | none | <content-list> ] [/ <string> ]?
Initial: normal
Applies to: ::before, ::after, ::marker, and page margin boxes. Image and url values can apply to all elements.
Inherited: no
Percentages: n/a
Media: all
Computed value: As specified below.
Animation type: discrete

The content property dictates what is rendered inside the element or pseudo-element. It takes a comma separated list of URIs, followed by a space separated list of tokens, followed by an optional slash and string indicating alternative text. If there are multiple URIs provided, then each is tried in turn until a value which is both available and supported is found. The last value is used as a fallback if the others fail.

normal
For an element, this computes to contents.

For ::before and ::after, this computes to inhibit.

none
On elements, this inhibits the children of the element from being rendered as children of this element, as if the element was empty.

On pseudo-elements it inhibits the creation of the pseudo-element as if it had display: none.

In neither case does it prevent any pseudo-elements which have this element or pseudo-element as an originating element from being generated.

<content-list>
[ <string> | contents | <uri> | <quote> | document-url | <datetime> | <target> | leader() ]+

See sections below for details on each of these.

should the contents keyword be replaced with content()

If the computed value of the part of the content property that ends up being used is a single URI, then the element or pseudo-element is a replaced element. The box model defines different rules for the layout of replaced elements than normal elements. Replaced elements do not have '::before' and '::after' pseudo-elements; the content property in the case of replaced content replaces the entire contents of the element’s box.

State that generated content should be searchable, selectable, and available to assistive technologies.

Generated content often needs alternative text. If the value of the content property is a string, this string should be used by assistive technology unless an alternative is provided. This allows purely decorative text to be ignored by providing the empty string as alternative text. Appropriate alternative text must be provided for images inserted via the content property as well.

Here the content property is an image, so the alt value is required to provide alternative text.
.new::before {
  content: url(./img/star.png) / "New!";
    /* or a localized attribute from the DOM: attr("data-alt") */
}
If the pseudo-element is purely decorative and its function is covered elsewhere, setting alt to the empty string can avoid reading out the decorative element. Here the ARIA attribute will be spoken as "collapsed". Without the empty string alt value, the content would also be spoken as "Black right-pointing pointer".
.expandable::before {
  content: "\25BA" / ""; 
/* a.k.a. ► */
  /* aria-expanded="false" already in DOM,
     so this pseudo-element is decorative */
}

2. Content Values and Functions

The content property can be used to add many different types of content to a document, including images, strings, the values of counters, and the text value of elements. In this section we enumerate the possibilities.

2.1. String

<string>
The element or pseudo-element contains the specified string. Occurrences of white space characters in the string are handled according to the properties given in the [CSS3TEXT] module.

2.2. URI

<uri>
For URIs other than URIs in the last comma separated section of the value, if the URI is available and the format is supported, then the element or pseudo-element becomes a replaced element, otherwise, the next item in the comma separated list is used, if any.
h1 { content: url(header/mng), url(header/png), none; }

In the example above, if header/mng wasn’t in a supported format, then header/png would have been used instead. In the example above, if header/png wasn’t available either, then the <h1> element would be empty, as the last alternative is none.

To make an element fallback on its contents, you have to explicitly give contents as a fallback:

content: url(1), url(2), url(3), contents;

What happens when no formats are supported, and the author does not explicitly indicate a fallback? Why doesn’t an element fallback to contents unless an author explicitly says so?

If the URI is part of the last comma separated value in the list, as the second URI in the following example:

h1 { content: url(welcome), "Welcome to: " url(logo); }

...then if the file is available and the format is supported, then an anonymous replaced inline element is inserted, otherwise the image is ignored (as if it hadn’t been given at all).

There appears to be some change from [CSS21] which says, "If the user agent cannot display the resource it must either leave it out as if it were not specified or display some indication that the resource cannot be displayed." Are we now saying that a user agent cannot display a missing image graphic in this situation?

When a URI is used as replaced content, it affects the generation of ::before and ::after pseudo-elements.

2.3. contents

contents
The element’s descendents. Since this can only be used once per element (you can’t duplicate the children if, e.g., one is a plugin or form control), it is handled as follows:

If set on the element:

Always honoured. Note that this is the default, since the initial value of content is normal and normal computes to contents on an element.

If set on one of the element’s other pseudo-elements:

Check to see that it is not set on a "previous" pseudo-element, in the following order, depth first:

  1. the element itself

  2. ::before

  3. ::after

Should this behave as an empty string on pseudo-elements?

If it is already used, then it evaluates to nothing (like none). Only pseudo-elements that are actually generated are checked.

In the following case:
foo { content: normal; }   /* this is the initial value */
foo::after { content: contents; }

...the element’s content property would compute to contents and the after pseudo element would have no contents (equivalent to none) and thus would not appear.

foo { content: none; }
foo::after { content: contents; }

But in this example, the ::after pseudo-element will contain the contents of the foo element.

Use cases for suppressing the content on the element and using it in a pseudo-element would be welcome.

Note that while it is useless to include contents twice in a single content property, that is not a parse error. The second occurrence simply has no effect, as it has already been used. It is also not a parse error to use it on a marker pseudo-element, it is only during the rendering stage that it gets treated like none.

Do we need the statement about marker pseudo-elements here? Or is this legacy from the old version of the spec?

2.4. Quotes

HTML has long had the q element, used to delimit quotations. The quotes property, in conjunction with the various *-quote values of the content property, can be used to properly style such quotations.

2.4.1. Specifying quotes with the quotes property

Name: quotes
Value: [ <string> <string> ]+ | none
Initial: depends on user agent
Applies to: all elements
Inherited: yes
Percentages: n/a
Media: all
Computed value: specified value
Animation type: discrete

The previous ED had an initial value of text, which was an error. [CSS21] has initial value of "depends on user agent". Do we use auto for things like this, or is it just a UA stylesheet issue?

This property specifies quotation marks for any number of embedded quotations. Values have the following meanings:

none
The open-quote and close-quote values of the content property produce no quotations marks, as if they were no-open-quote and no-close-quote respectively.
[ <string> <string> ]+
Values for the open-quote and close-quote values of the content property are taken from this list of pairs of quotation marks (opening and closing). The first (leftmost) pair represents the outermost level of quotation, the second pair the first level of embedding, etc. The user agent must apply the appropriate pair of quotation marks according to the level of embedding.

2.4.2. The *-quote values of the content property

<quote> = [open-quote | close-quote | no-open-quote | no-close-quote] )
open-quote
close-quote
These values are replaced by the appropriate string from the quotes property, and increments (decrements) the level of nesting for quotes. See §2.4.1 Specifying quotes with the quotes property for more information.
no-open-quote
no-close-quote
Inserts nothing (as in none), but increments (decrements) the level of nesting for quotes. See §2.4.1 Specifying quotes with the quotes property for more information.

Quotation marks are inserted in appropriate places in a document with the open-quote and close-quote values of the content property. Each occurrence of open-quote or close-quote is replaced by one of the strings from the value of quotes, based on the depth of nesting.

open-quote refers to the first of a pair of quotes, close-quote refers to the second. Which pair of quotes is used depends on the nesting level of quotes: the number of occurrences of open-quote in all generated text before the current occurrence, minus the number of occurrences of close-quote. If the depth is 0, the first pair is used, if the depth is 1, the second pair is used, etc. If the depth is greater than the number of pairs, the last pair is repeated.

Note that this quoting depth is independent of the nesting of the source document or the formatting structure.

Some typographic styles require open quotation marks to be repeated before every paragraph of a quote spanning several paragraphs, but only the last paragraph ends with a closing quotation mark. In CSS, this can be achieved by inserting "phantom" closing quotes. The keyword no-close-quote decrements the quoting level, but does not insert a quotation mark.

The following style sheet puts opening quotation marks on every paragraph in a blockquote, and inserts a single closing quote at the end:
blockquote p:before { content: open-quote }
blockquote p:after { content: no-close-quote }
blockquote p:last-child::after { content: close-quote }

For symmetry, there is also a no-open-quote keyword, which inserts nothing, but increments the quotation depth by one.

If a quotation is in a different language than the surrounding text, it is customary to quote the text with the quote marks of the language of the surrounding text, not the language of the quotation itself.
For example, French inside English:
The device of the order of the garter is “Honi soit qui mal y pense.”

English inside French:

Il disait: « Il faut mettre l’action en ‹ fast forward ›. »

A style sheet like the following will set the quotes property so that open-quote and close-quote will work correctly on all elements. These rules are for documents that contain only English, French, or both. One rule is needed for every additional language. Note the use of the child combinator (">") to set quotes on elements based on the language of the surrounding text:

:lang(fr) > * { quotes: "\00AB\2005" "\2005\00BB" "\2039\2005" "\2005\203A" }
:lang(en) > * { quotes: "\201C" "\201D" "\2018" "\2019" }

The quotation marks are shown here in a form that most people will be able to type. If you can type them directly, they will look like this:

:lang(fr) > * { quotes: "« " " »" "‹ " " ›" }
:lang(en) > * { quotes: "“" "”" "‘" "’" }

For example, applying the following style sheet:

/* Specify pairs of quotes for two levels in two languages */
:lang(en) > q { quotes: '"' '"' "'" "'" }
:lang(no) > q { quotes: "«" "»" "’" "’" }

/* Insert quotes before and after Q element content */
q::before { content: open-quote }
q::after  { content: close-quote }

to the following HTML fragment:

<html lang="en">
  <head>
    <title>Quotes</title>
  </head>
  <body>
    <p><q>Quote me!</q></p>
  </body>
</html>

would allow a user agent to produce:

"Quote me!"

while this HTML fragment:

<html lang="no">
  <head>
    <title>Quotes</title>
  </head>
  <body>
    <p><q>Trøndere gråter når <q>Vinsjan på kaia</q> blir deklamert.</q></p>
  </body>
</html>

would produce:

«Trøndere gråter når ’Vinsjan på kaia’ blir deklamert.»

2.5. Leaders

A leader, sometimes known as a tab leader or a dot leader, is a repeating pattern used to visually connect content across horizontal spaces. They are most commonly used in tables of contents, between titles and page numbers. The leader() function, as a value for the content property, is used to create leaders in CSS. This function takes a string (the leader string), which describes the repeating pattern for the leader.

2.5.1. The leader() function

leader()
Inserts a leader. See the section on leaders for more information.
leader() = leader( dotted | solid | space | <string>);

Three keywords are shorthand values for common strings:

dotted
Equivalent to leader(".")
solid
Equivalent to leader("_")
space
Equivalent to leader(" ")
<string>
Issue: Define this.
ol.toc a::after {
  content: leader('.') target-counter(attr(href), page);
}

<h1>Table of Contents</h1>
<ol class="toc">
<li><a href="#chapter1">Loomings</a></li>
<li><a href="#chapter2">The Carpet-Bag</a></li>
<li><a href="#chapter3">The Spouter-Inn</a></li>
</ol>

This might result in:

Table of Contents

1. Loomings.....................1
2. The Carpet-Bag...............9
3. The Spouter-Inn.............13

Do leaders depend on the assumption that the content after the leader is right-aligned (end-aligned)?

2.5.2. Rendering leaders

Consider a line which contains the content before the leader (the “before content”), the leader, and the content after the leader (the “after content”). Leaders obey the following rules:

  1. The leader string must appear in full at least once.
  2. The leader should be as long as possible
  3. Visible characters in leaders should vertically align with each other when possible.
  4. Line break characters in the leader string must be ignored.
  5. White space in the leader string follows normal CSS rules.
  6. A leader only appears between the start content and the end content.
  7. A leader only appears on a single line, even if the before content and after content are on different lines.
  8. A leader can’t be the only thing on a line.

2.5.3. Procedure for rendering leaders

  1. Lay out the before content, until reaching the line where the before content ends.
    BBBBBBBBBB
    BBB
    
  2. The leader string consists of one or more glyphs, and is thus an inline box. A leader is a row of these boxes, drawn from the end edge to the start edge, where only those boxes not overlaid by the before or after content. On this line, draw the leader string, starting from the end edge, repeating as many times as possible until reaching the start edge.
    BBBBBBBBBB
    ..........
    
  3. Draw the before and after content on top of the leader. If any part of the before or after content overlaps a glyph in a leader string box, that glyph is not displayed.
    BBBBBBBBBB
    BBB....AAA
    
  4. If one full copy of the leader string is not visible:
    BBBBBBB
    BBBBBBA
    
    Insert a line break after the before content, draw the leader on the next line, and draw the end content on top, and hide any leader strings that are not fully displayed.
    BBBBBBB
    BBBBBB
    ......A
    

what to do if after content is wider than the line box?

Leaders don’t quite work in table layouts. How can we fix this?

drawing leaders
Procedure for drawing leaders
drawing leaders
Procedure for drawing leaders when the content doesn’t fit on a single line

2.6. Cross references and the target-* functions

Many documents contain internal references:

Three new values for the content property are used to automatically create these types of cross-references: target-counter(), target-counters(), and target-text(). Each of these displays information obtained from the target end of a link.

2.6.1. The target-counter() function

target-counter() = target-counter( [ <string> | <url> ] , <custom-ident> [ , <counter-style> ]? )

The target-counter() function retrieves the value of the innermost counter with a given name. The required arguments are the url of the target and the name of the counter. An optional counter-style argument can be used to format the result.

These functions only take a fragment URL which points to a location in the current document. If there’s no fragment, if the ID referenced isn’t there, or if the URL points to an outside document, the user agent must treat that as an error.

what should error handling be?

restrict syntactically to local references for now.

HTML:
…which will be discussed on page <a href="#chapter4_sec2"></a>.

CSS:

a::after { content: target-counter(attr(href url), page) }

Result:

…which will be discussed on page 137.
Page numbers in tables of contents can be generated automatically:

HTML:

<nav>
  <ol>
    <li class="frontmatter"><a href="#pref_01">Preface</a></li>
    <li class="frontmatter"><a href="#intr_01">Introduction</a></li>
    <li class="bodymatter"><a href="#chap_01">Chapter One</a></li>
  </ol>
</nav>

CSS:

.frontmatter a::after { content: leader('.') target-counter(attr(href url), page, lower-roman) }
.bodymatter a::after { content: leader('.') target-counter(attr(href url), page, decimal) }

Result:

Preface.............vii
Introduction.........xi
Chapter One...........1

2.6.2. The target-counters() function

This functions fetches the value of all counters of a given name from the end of a link, and formats them by inserting a given string between the value of each nested counter.
target-counters() = target-counter( [ <string> | <url> ] , <custom-ident> , <string> [ , <counter-style> ]? )
I have not found a compelling example for target-counters() yet.

found a compelling example, in CSS specs. Do something.

2.6.3. The target-text() function

The target-text() function retrieves the text value of the element referred to by the URL. An optional second argument specifies what content is retrieved, using the same values as the string-set property above.
target-text() = target-text( [ <string> | <url> ] [ , [ content | before | after | first-letter] ]? )

A simpler syntax has been proposed by fantasai: http://lists.w3.org/Archives/Public/www-style/2012Feb/0745.html

…which will be discussed <a href="#chapter_h1_1">later</a>.

a::after { content: ", in the chapter entitled " target-text(attr(href url)) }

Result: …which will be discussed later, in the chapter entitled Loomings.

2.7. Named strings

This section introduces named strings, which are the textual equivalent of counters and which have a distinct namespace from counters. Named strings follow the same nesting rules as counters. The string-set property accepts values similar to the content property, including the extraction of the current value of counters.

Named strings are a convenient way to pull metadata out of the document for insertion into headers and footers. In HTML, for example, META elements contained in the document HEAD can set the value of named strings. In conjunction with attribute selectors, this can be a powerful mechanism:

meta[author] { string-set: author attr(author); }
head > title { string-set: title contents; }
@page:left {
  @top {
    text-align: left;
    vertical-align: middle;
    content: string(title);
  }
}
@page:right {
  @top {
    text-align: right;
    vertical-align: middle;
    content: string(author);
  }
}

2.7.1. The string-set property

Name: string-set
Value: [ <custom-ident> <string>+ ]# | none
Initial: none
Applies to: all elements, but not pseudo-elements
Inherited: no
Percentages: N/A
Media: all
Computed value: specified value
Animation type: discrete

The string-set property copies the text content of an element into a named string, which functions as a variable. The text content of this named string can be retrieved using the string() function. Since these variables may change on a given page, an optional second value for the string() function allows authors to choose which value on a page is used.

none
The element does not set any named strings.
[ <custom-ident> <string>+ ]#
The element establishes one or more named strings, corresponding to each comma-separated entry in the list.

For each entry, the <custom-ident> gives the name of the named string. It’s followed by one or more <string> values, which are concatenated together to form the value of the named string.

The following example captures the contents of H1 elements, which represent chapter names in this hypothetical document.
H1 { string-set: chapter contents; }

When an H1 element is encountered, the chapter string is set to the element’s textual contents, and the previous value of chapter, if any, is overwritten.

2.7.2. The string() function

string() = string( <custom-ident> , [ first | start | last | first-except ]? )

The string() function is used to copy the value of a named string to the document, via the content property. This function requires one argument, the name of the named string. Since the value of a named string may change several times on a page (as multiple elements defining the string can appear) an optional second argument indicates which value of the named string should be used.

The second argument of the string() function is one of the following keywords:

first
The value of the first assignment on the page is used. If there is no assignment on the page, the entry value is used. If no second argument is provided, this is the default value.
start
If the element is the first element on the page, the value of the first assignment is used. Otherwise the entry value is used. The entry value may be empty if the named string hasn’t yet appeared.
last
The exit value of the named string is used.
first-except
This is identical to first, except that the empty string is used on the page where the value is assigned.

we may need to kill the entire content string. Is this necessary?

The content values of named strings are assigned at the point when the content box of the element is first created (or would have been created if the element’s display value is none). The entry value for a page is the assignment in effect at the end of the previous page. The exit value for a page is the assignment in effect at the end of the current page.

CSS:
@page {
   size: 15cm 10cm;
   margin: 1.5cm;

   @top-left {
   content: "first: " string(heading, first);
   }
   @top-center {
   content: "start: " string(heading, start);
   }
    @top-right {
    content: "last: " string(heading, last);
   }
  }

h2 { string-set: heading content() }

The following figures show the first, start, and last assignments of the “heading” string on various pages.

The start value is empty, as the string had not yet been set at the start of the page.
Since the page starts with an h2, the start value is the value of that head.
Since there’s not an h2 at the top of this page, the start value is the exit value of the previous page.

2.7.3. The content() function

content() = content( [text | before | after | first-letter | marker ]? )
text
The string value of the element. If no value is specified in content(), it acts as if text were specified.
before
The string value of the ::before pseudo-element.
after
The string value of the ::after pseudo-element.
first-letter
The first letter of the element, as defined for the ::first-letter pseudo-element
marker
The string value of the ::marker pseudo-element.
HTML:
<h1>Loomings</h1>

CSS:

h1::before { content: 'Chapter ' counter(chapter); }
h1 { string-set: header content(before) ':' content(text); }
h1::after { content: '.'; }

The value of the named string “header” will be “Chapter 1: Loomings”.

HTML:
<section title="Loomings">

CSS:

section { string-set: header attr(title) }

The value of the “header” string will be “Loomings”.

3. Automatic counters and numbering: the counter-increment and counter-reset properties (moved)

Now described in [CSS3LIST]

Should this move back to CSS Content?

4. Bookmarks

Some document formats, most notably PDF, allow the use of bookmarks as an aid to navigation. Bookmarks provide a hierarchy of links to document elements, as well as text to label the links. A bookmark has three properties: bookmark-level, bookmark-label, and bookmark-state.

A bookmark references that element in the document. When a user activates a bookmark, the user agent must bring that reference point to the user’s attention, exactly as if navigating to that element by fragment URL. This will also trigger matching the :target pseudo-class.

4.1. bookmark-level

The bookmark-level property determines if a bookmark is created, and at what level. If this property is absent, or has value none, no bookmark should be generated, regardless of the values of bookmark-label or bookmark-state.

Name: bookmark-level
Value: none | <integer>
Initial: none
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Animation type: discrete
<integer>
defines the level of the bookmark, with the highest level being 1 (negative and zero values are invalid).
none
no bookmark is generated.
section h1 { bookmark-level: 1; }
section section h1 { bookmark-level: 2; }
section section section h1 { bookmark-level: 3; }

4.2. bookmark-label

Name: bookmark-label
Value: <content-list>
Initial: content(text)
Applies to: all elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Animation type: discrete
<content-list>
<content-list> is defined above, in the section on the string-set property. The value of <content-list> becomes the text content of the bookmark label.
HTML:
<h1>Loomings</h1>

CSS:

h1 {
bookmark-label: content(text);
bookmark-level: 1;
}

The bookmark label will be “Loomings”.

4.3. bookmark-state

Bookmarks typically toggle open and closed, with the open state displaying the next level of bookmarks.
Name: bookmark-state
Value: open | closed
Initial: open
Applies to: block-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: specified value
Animation type: discrete
open
The bookmarks of the nearest descendants of an element with a bookmark-state of open will be displayed.
closed
any bookmarks of descendant elements are not initially displayed.

is the initial bookmark state, or the bookmark state updated by the UA as appropriate?

5. Changes since the 14 May 2003 Working Draft

Acknowledgments

Stuart Ballard, David Baron, Bert Bos, and Tantek Çelik provided invaluable suggestions used in this specification.

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

Terms defined by reference

References

Normative References

[CSS-CONTAINMENT-3]
CSS Containment Module Level 3 URL: https://www.w3.org/TR/css-containment-3/
[CSS-COUNTER-STYLES-3]
Tab Atkins Jr.. CSS Counter Styles Level 3. 11 June 2015. CR. URL: http://www.w3.org/TR/css-counter-styles-3/
[CSS-IMAGES-3]
CSS Image Values and Replaced Content Module Level 3 URL: https://www.w3.org/TR/css3-images/
[CSS-PSEUDO-4]
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 15 January 2015. WD. URL: http://www.w3.org/TR/css-pseudo-4/
[CSS-VALUES]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 11 June 2015. CR. URL: http://www.w3.org/TR/css-values/
[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: http://www.w3.org/TR/CSS2
[CSS3LIST]
Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 20 March 2014. WD. URL: http://www.w3.org/TR/css-lists-3/
[CSS3TEXT]
Elika Etemad; Koji Ishii. CSS Text Module Level 3. 10 October 2013. LCWD. URL: http://www.w3.org/TR/css-text-3/
[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://www.w3.org/TR/selectors4/

Informative References

[CSS3GCPM]
Dave Cramer. CSS Generated Content for Paged Media Module. 13 May 2014. WD. URL: http://www.w3.org/TR/css-gcpm-3/

Property Index

Name Value Initial Applies to Inh. %ages Media Anim­ation type Com­puted value
content [ [ <image> | <url> ] ',' ]* [ normal | none | <content-list> ] [/ <string> ]? normal ::before, ::after, ::marker, and page margin boxes. Image and url values can apply to all elements. no n/a all discrete As specified below.
quotes [ <string> <string> ]+ | none depends on user agent all elements yes n/a all discrete specified value
string-set [ <custom-ident> <string>+ ]# | none none all elements, but not pseudo-elements no N/A all discrete specified value
bookmark-level none | <integer> none all elements no N/A visual discrete specified value
bookmark-label <content-list> content(text) all elements no N/A visual discrete specified value
bookmark-state open | closed open block-level elements no N/A visual discrete specified value

Issues Index

should the contents keyword be replaced with content()
State that generated content should be searchable, selectable, and available to assistive technologies.
What happens when no formats are supported, and the author does not explicitly indicate a fallback? Why doesn’t an element fallback to contents unless an author explicitly says so?
There appears to be some change from [CSS21] which says, "If the user agent cannot display the resource it must either leave it out as if it were not specified or display some indication that the resource cannot be displayed." Are we now saying that a user agent cannot display a missing image graphic in this situation?
Should this behave as an empty string on pseudo-elements?
Use cases for suppressing the content on the element and using it in a pseudo-element would be welcome.
Do we need the statement about marker pseudo-elements here? Or is this legacy from the old version of the spec?
The previous ED had an initial value of text, which was an error. [CSS21] has initial value of "depends on user agent". Do we use auto for things like this, or is it just a UA stylesheet issue?
Do leaders depend on the assumption that the content after the leader is right-aligned (end-aligned)?
what to do if after content is wider than the line box?
Leaders don’t quite work in table layouts. How can we fix this?
what should error handling be?
restrict syntactically to local references for now.
found a compelling example, in CSS specs. Do something.
A simpler syntax has been proposed by fantasai: http://lists.w3.org/Archives/Public/www-style/2012Feb/0745.html
we may need to kill the entire content string. Is this necessary?
Now described in [CSS3LIST]
Should this move back to CSS Content?
is the initial bookmark state, or the bookmark state updated by the UA as appropriate?