CSS Pseudo-Elements Module Level 4

1. Introduction

This section is informative.

Pseudo-elements represent abstract elements of the document beyond those elements explicitly created by the document language. Since they are not restricted to fitting into the document tree, the can be used the select and style portions of the document that do not necessarily map to the document’s tree structure. For instance, the ::first-line pseudo-element can select content on the first formatted line of an element after text wrapping, allowing just that line to be styled differently from the rest of the paragraph.

Each pseudo-element is associated with an originating element and has syntax of the form ::name-of-pseudo. This module defines the pseudo-elements that exist in CSS and how they can be styled. For more information on pseudo-elements in general, and on their syntax and interaction with other selectors, see [SELECTORS4].

2. Typographic Pseudo-elements

2.1. The ::first-line pseudo-element

The ::first-line pseudo-element describes the contents of the first formatted line of its originating element.

The rule below means “change the letters of the first line of every p element to uppercase”:

p::first-line { text-transform: uppercase }

The selector p::first-line does not match any real document element. It does match a pseudo-element that conforming user agents will insert at the beginning of every p element.

Note: Note that the length of the first line depends on a number of factors, including the width of the page, the font size, etc.

For example, given an ordinary HTML [HTML5] paragraph such as:

<P>This is a somewhat long HTML
paragraph that will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

The lines might be broken as follows:

THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag
sequence. The other lines will be treated as
ordinary lines in the paragraph.

This paragraph might be “rewritten” by user agents to include a fictional tag sequence to represent ::first-line. This fictional tag sequence helps to show how properties are inherited.

<P><P::first-line> This is a somewhat long HTML
paragraph that </P::first-line> will be broken into several
lines. The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

If a pseudo-element breaks up a real element, the desired effect can often be described by a fictional tag sequence that closes and then re-opens the element.

Thus, if we mark up the previous paragraph with a span element encompassing the first sentence:

<P><SPAN class="test"> This is a somewhat long HTML
paragraph that will be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

the user agent could simulate start and end tags for span when inserting the fictional tag sequence for ::first-line to get the correct inheritance behavior.

<P><P::first-line><SPAN class="test"> This is a somewhat long HTML
paragraph that will </SPAN></P::first-line><SPAN class="test"> be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the
paragraph.</P>

2.1.1. Finding the First Formatted Line

In CSS, the ::first-line pseudo-element can only have an effect when attached to a block container. The first formatted line of an element must occur inside a block-level descendant in the same flow (i.e., a block-level descendant that is not out-of-flow due to floating or positioning).

For example, the first line of the DIV in <DIV><P>This line...</P></DIV> is the first line of the P (assuming that both P and DIV are blocks).

The first line of a table-cell or inline-block cannot be the first formatted line of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first formatted line of the DIV is not the line "Hello".

Note: Note that the first line of the p in this fragment: <p><br>First... doesn’t contain any letters (assuming the default style for br). The word "First" is not on the first formatted line.

A user agent must act as if the fictional start tags of a ::first-line pseudo-element were nested just inside the innermost enclosing block-level element.

For example, the fictional tag sequence for

<DIV>
  <P>First paragraph</P>
  <P>Second paragraph</P>
</DIV>

<DIV>
  <P><DIV::first-line><P::first-line>First paragraph</P::first-line></DIV::first-line></P>
  <P><P::first-line>Second paragraph</P::first-line></P>
</DIV>

2.1.2. Styling the First Line Pseudo-element

The ::first-line pseudo-element’s generated box behaves similar to that of an inline-level element, but with certain restrictions. The following CSS properties apply to a ::first-line pseudo-element:

all font properties (see [CSS3-FONTS])
the color and opacity properties (see [CSS3COLOR])
all background properties (see [CSS3BG])
any typesetting properties that apply to inline elements (see [CSS3TEXT])
all text decoration properties (see [CSS3-TEXT-DECOR])
any inline layout properties that apply to inline elements (see [CSS3LINE])
any other properties defined to apply to ::first-line by their respective specifications

User agents may apply other properties as well.

2.1.3. Inheritance and the ::first-line Pseudo-element

During CSS inheritance, the portion of a child element that occurs on the first line only inherits properties applicable to the ::first-line pseudo-element from the ::first-line pseudo-element. For all other properties inheritence is from the non-pseudo-element parent of the first line pseudo element. (The portion of a child element that does not occur on the first line always inherits from the parent of that child.)

2.2. The ::first-letter pseudo-element

The ::first-letter pseudo-element represents the first typographic letter unit [CSS3TEXT] on the first formatted line of its originating element, if it is not preceded by any other content (such as images or inline tables) on its line. The ::first-letter pseudo-element can be used to create “initial caps” and “drop caps”, which are common typographic effects.

For example, the following rule creates a 2-line drop-letter on every paragraph following a level-2 header, using the initial-letter property defined in [CSS3LINE]:

h2 + p::first-letter { initial-letter: 2; }

Punctuation (i.e, characters that belong to the Punctuation (P*) Unicode general category [UAX44]) that precedes or follows the first typographic letter unit must also be included in the ::first-letter pseudo-element.

As explained in [CSS3TEXT], a typographic letter unit can include more than one Unicode codepoint. For example, combining characters must be kept with their base character. Also, languages may have additional rules about how to treat certain letter combinations. In Dutch, for example, if the letter combination "ij" appears at the beginning of an element, both letters should be considered within the ::first-letter pseudo-element. [UAX29] The UA should tailor its definition of typographic letter unit to reflect the first-letter traditions of the originating element’s content language.

This is actually a problem in cases where the originating element is an ancestor with a different content. What should we say here?

Note: Note that the first typographic letter unit may in fact be a digit, e.g., the “6” in “67 million dollars is a lot of money.”

If the characters that would form the ::first-letter are not in the same element, such as "‘T" in <p>'<em>T..., the user agent may create a ::first-letter pseudo-element from one of the elements, both elements, or simply not create a pseudo-element. Additionally, if the first letter(s) of the block are not at the start of the line (for example due to bidirectional reordering), then the user agent need not create the pseudo-element(s).

The ::first-letter pseudo-element is contained within any ::first-line pseudo-elements, and thus inherits from ::first-line.

2.2.1. Finding the First Letter

The first letter must occur on the first formatted line. For example, in this HTML fragment: <p><br>First... the first line doesn’t contain any letters and ::first-letter doesn’t match anything. In particular, it does not match the “F” of “First”.

In CSS, the ::first-letter pseudo-element only applies to block containers. A future version of this specification may allow this pseudo-element to apply to more display types. The ::first-letter pseudo-element can be used with all such elements that contain text, or that have a descendant in the same flow that contains text. A user agent should act as if the fictional start tag of the ::first-letter pseudo-element is just before the first text of the element, even if that first text is in a descendant.

Example: The fictional tag sequence for this HTML fragment:

<div>
<p>The first text.

is:

<div>
<p><div::first-letter><p::first-letter>T</...></...>he first text.

In CSS the first letter of a table-cell or inline-block cannot be the first letter of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first letter of the DIV is not the letter "H". In fact, the DIV doesn’t have a first letter. If an element is a list item (display: list-item), the ::first-letter applies to the first letter in the principal box after the marker. User-Agents may ignore ::first-letter on list items with list-style-position: inside. If an element has ::before or ::after content, the ::first-letter applies to the first letter of the element including that content.

Example: After the rule p::before {content: "Note: "}, the selector p::first-letter matches the "N" of "Note".

2.2.2. Styling the ::first-letter Pseudo-element

In CSS a ::first-letter pseudo-element is similar to an inline-level element. The following properties that apply to ::first-letter pseudo-elements:

all font properties (see [CSS3-FONTS])
the color and opacity properties (see [CSS3COLOR])
all background properties (see [CSS3BG])
any typesetting properties that apply to inline elements (see [CSS3TEXT])
all text decoration properties (see [CSS3-TEXT-DECOR])
any inline layout properties that apply to inline elements (see [CSS3LINE])
margin and padding properties (see [CSS21])
border properties and box-shadow (see [CSS3BG])
any other properties defined to apply to ::first-letter by their respective specifications

User agents may apply other properties as well.

Note: In previous levels of CSS, User Agents were allowed to choose a line height, width and height based on the shape of the letter, approximate font sizes, or to take the glyph outline into account when formatting. This possibility has been intentionally removed, as it proved to be a poor solution for the intended use case (Drop Caps), yet caused interoperability problems.

Example: This CSS and HTML example shows a possible rendering of an initial cap. Note that the fictional start tag of the first letter is inside the span, and thus the font weight of the first letter is normal, not bold as the span:

p { line-height: 1.1 }
p::first-letter { font-size: 3em; font-weight: normal }
span { font-weight: bold }
...
<p><span>Het hemelsche</span> gerecht heeft zich ten lange lesten<br>
Erbarremt over my en mijn benaeuwde vesten<br>
En arme burgery, en op mijn volcx gebed<br>
En dagelix geschrey de bange stad ontzet.

The following CSS will make a drop cap initial letter span about two lines:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
 <HEAD>
  <TITLE>Drop cap initial letter</TITLE>
  <STYLE type="text/css">
   P               { font-size: 12pt; line-height: 1.2 }
   P::first-letter { font-size: 200%; font-weight: bold; float: left }
   SPAN            { text-transform: uppercase }
  </STYLE>
 </HEAD>
 <BODY>
  <P><SPAN>The first</SPAN> few words of an article
    in The Economist.</P>
 </BODY>
</HTML>

This example might be formatted as follows:

The fictional tag sequence is:

<P>
  <SPAN>
    <P::first-letter>
      T
    </P::first-letter>he first
  </SPAN>
  few words of an article in the Economist.
</P>

Note that the ::first-letter pseudo-element tags abut the content (i.e., the initial character), while the ::first-line pseudo-element start tag is inserted right after the start tag of the block element.

3. Highlight Pseudo-elements

3.1. Selecting Highlighted Content: the ::selection, ::inactive-selection, ::spelling-error, and ::grammar-error pseudo-elements

The highlight pseudo-elements represent portions of a document that have been highlighted in some way.

::selection
::inactive-selection: The ::selection and ::inactive-selection pseudo-elements represent the portion of a document that has been highlighted by the user. This also applies, for example, to selected text within an editable text field.
::selection applies to active selections, wherease ::inactive-selection applies to inactive selections (e.g. when the document window is inactive and therefore not receiving events).
::spelling-error: The ::spelling-error pseudo-element represents a portion of text that has been flagged by the user agent as misspelled.
::grammar-error: The ::grammar-error pseudo-element represents a portion of text that has been flagged by the user agent as grammatically incorrect.

Note: A future level of CSS may introduce ways to create custom highlight pseudo-elements.

3.2. Styling Highlights

The following properties apply to the highlight pseudo-elements:

color
background-color
cursor
caret-color
outline and its longhands
text-decoration and its associated properties
text-emphasis-color
text-shadow

Are there any other properties that should be included here?

Note: Historically (and at the time of writing) only color and background-color have been interoperably supported.

If color is not specified, the text (and text decoration)'s unselected color must be used for the highlighted text. (As usual, the initial background-color is transparent.) Issue: Can we reuse currentColor for this, now that it computes to itself?

The UA should use the OS-default highlight colors when neither color nor background-color has been specified by the author.

Note: This paired-cascading behavior does not allow using the normal cascade to represent the OS default selection colors. However it has been interoperably implemented in browsers and is thus probably a Web-compatibility requirement.

The color property specifies the color of both the text and all line decorations (underline, overline, line-through) applied to the text. Note: Because text-emphasis-color defaults to currrentColor, it will by default also set the color of the the emphasis marks.

3.3. Area of a Highlight

For each type of highlighting (see §3.1 Selecting Highlighted Content: the ::selection, ::inactive-selection, ::spelling-error, and ::grammar-error pseudo-elements) there exists a single highlight overlay for the entire document, the active portions of which are represented by the corresponding highlight pseudo-element. Each box owns the piece of the overlay corresponding to any text or replaced content directly contained by the box.

For text, the corresponding overlay must cover at least the entire em box and may extend further above/below the em box to the line box edges. Spacing between two characters may also be part of the overlay area, in which case it belongs to the innermost element that contains both characters and is selected when both characters are selected.
For replaced content, the associated overlay must cover at least the entire replaced object, and may extend outward to include the element’s entire content box.
The overlay may also include other other areas within the border-box of an element; in this case, those areas belong to the innermost such element that contains the area.

See F2F minutes, dbaron’s message, Daniel’s thread, Gecko notes, Opera notes, Webkit notes

Not sure if this is the correct way of describing the way things work.

3.4. Cascading and Per-Element Highlight Styles

Each element draws its own portion of the highlight overlay, which receives the styles specified by the highlight pseudo-element styles for which that element or one of its ancestors is the originating element. When multiple styles conflict, the winning style is the one belonging to the innermost element after cascading.

For example, if the following rules were applied:

p::selection      { color: yellow; background: green; }
p > em::selection { color: orange; }
em::selection     { color:    red; }

to the following markup:

<p>Highlight this <em>and this>/em>.</p>

The highlight would be green throughout, with yellow text outside the <em> element and orange text inside it.

This could alternately be described in terms of inheritance. The observable differences would be in how inherit and unset behave. Should it inherit from the parent ::selection or the originating element? Opera does the former, Gecko/Blink the latter.

Authors wanting multiple selections styles should use :root::selection for their document-wide selection style, since this will allow clean overriding in descendants. ::selection alone applies to every element in the tree, overriding the more specific styles of any ancestors.

For example, if an author specified

::selection { background: blue; }
p.warning   { background:  red; }

and the document included

<p class="warning">Some <strong>very important information</strong></p>

The highlight would be blue around “very important information” because the <strong> element´s ::selection also matches the ::selection { background: blue; } rule. (Remember that * is implied when a tag selector is missing.) The style rules that would give the intended behavior are

:root::selection { background: blue; }
p.warning        { background:  red; }

3.5. Painting the Highlight

Each highlight pseudo-element draws its background over the active portion of the overlay, painting it immediately below any positioned descendants (i.e. just before step 8 in CSS2.1§E.2). It also suppresses the drawing of any selected text (and any text decorations applied to that text) and instead redraws that text (and its decorations) over the highlight overlay background using the specified color.

What should happen with text shadows? Drawing them in their original color is disconcerting if that color is not a shade of gray.

For non-replaced content, the UA must honor the color and background-color (including their alpha channels) as specified. However, for replaced content, the UA should create a semi-transparent wash to coat the content so that it can show through the selection. This wash should be of the specified background-color if that is not transparent, else of the specified color; however the UA may adjust the alpha channel if it is opaque.

The ::selection overlay is drawn over the ::spelling-error overlay which is drawn over the ::grammar-error overlay.

3.6. Security and Privacy Considerations

Because the styling of spelling and grammar errors can leak information about the contents of a user’s dictionary (which can include the user’s name and even includes the contents of his/her address book!) UAs that implement ::spelling-error and ::grammar-error must prevent pages from being able to read the styling of such highlighted segments.

4. Tree-Abiding Pseudo-elements

These pseudo-elements always fit within the box tree.

4.1. Generated Content Pseudo-elements: ::before and ::after

When their computed content value is not none, these pseudo-elements generate boxes as if they were immediate children of their originating element, and can be styled exactly like any normal document-sourced element in the document tree. They inherit any inheritable properties from their originating element; non-inheritable properties take their initial values as usual. [CSS3CASCADE]

::before: Represents a styleable child pseudo-element immediately before the originating element’s actual content.
::after: Represents a styleable child pseudo-element immediately after the originating element’s actual content.

For example, the following rule inserts the string “Note: ” before the content of every <p> element whose class attribute has the value note:

p.note::before { content: "Note: " }

Since the initial value of display is inline, this will generate an inline box. Like other inline children of <p>, it will participate in <p>’s inline formatting context, potentially sharing a line with other content.

As with the content of regular elements, the generated content of ::before and :after pseudo-elements may be included in any ::first-line and ::first-letter pseudo-elements applied to its originating element.

For compatibility with existing style sheets written against CSS Level 2 [CSS21], user agents must also accept the previous one-colon notation (:before and :after) for these pseudo-elements.

4.2. List Markers: the ::marker pseudo-element

The ::marker pseudo-element represents the automatically generated marker box of a list item. (See display: list-item.)

The following CSS properties apply to a ::marker pseudo-element:

all font properties (see [CSS3-FONTS])
the color property (see [CSS3COLOR])
the text-combine-upright property (see [CSS3-WRITING-MODES])

It is expected that future specifications will extend this list of properties; however at the moment marker box layout is not fully defined, so only these properties are allowed.

UAs must add the following rule to their default style sheet:

::marker { font-variant-numeric: tabular-nums; }

4.3. Placeholder Input: the ::placeholder pseudo-element

The ::placeholder pseudo-element represents placeholder text in an input field: text that represents the input and provides a hint to the user on how to fill out the form. For example, a date-input field might have the placeholder text “YYYY/MM/DD” to clarify that numeric dates are to be entered in year-month-day order.

All properties that apply to the ::first-line pseudo-element also apply to the ::placeholder pseudo-element.

In interactive media, placeholder text is often hidden once the user has entered input; however this is not a requirement, and both the input value and the placeholder text may be visible simultaneously. The exact behavior is UA-defined. Note that in static media (such as print) placeholder text will be present even after the user has entered input.

Current bikeshedding, based on implementations and discussions, appears to select from the list ::placeholder, ::placeholder-text, ::input-placeholder. The WG is going with ::placeholder. If there are other opinions that provide some kind of argument to the contrary, they should go to [email protected].

Authors seem to want text-align on the list of supported properties. See e.g. comments here.

5. Overlapping Pseudo-element Interactions

Recall that

the contents of ::before and ::after are selected exactly as if they were normal elements in the document source tree
the ::first-letter boundaries are tightly wrapped around the first letter text, and ::first-letter is constrained to exist solely on the first formatted line.
the ::first-line start is inserted inserted just inside the containing block’s element boundary, and its end after the close of all content on the line

The following CSS and HTML example illustrates how overlapping pseudo-elements interact:

p { color: red; font-size: 12pt }
p::first-letter { color: green; font-size: 200% }
p::first-line { color: blue }

<P>Some text that ends up on two lines</P>

The first letter of each P element will be green with a font size of ’24pt'. The rest of the first formatted line will be blue while the rest of the paragraph will be red.

Assuming that a line break will occur before the word "ends", the fictional tag sequence for this fragment might be:

<P>
<P::first-line>
<P::first-letter>
S
</P::first-letter>ome text that
</P::first-line>
ends up on two lines
</P>

6. Additions to the CSS Object Model

Pseudo-elements should be reachable by script, stylable from script, and available as event targets.

Note We may extend this section in the future to allow creation of pseudo-elements from script.

This entire section is a starting point for discussion. Feedback is welcome. Implementations, at this point, are not, as we are not yet sure of our approach.

This needs to be merged with the alternative PseudoElement and pseudo() proposal.

6.1. Interface CSSPseudoElement

The CSSPseudoElement interface allows pseudo-elements to be styleable from script and makes them event targets.

The approach in this draft is to start with a bare minimum for the CSSPseudoElement interface and build up from there. Another more radical approach could take everything that’s common between a pseudo-element and a node and create a new base class for both Node and CSSPseudoElement.

interface CSSPseudoElement {
    readonly attribute DOMString type;
    readonly attribute CSSStyleDeclaration style;
};

CSSPseudoElement implements EventTarget;

The type attribute is a string representing the type of the pseudo-element. This can be one of the following values:

‘before’: The pseudo-element was created before the element’s contents.
‘after’: The pseudo-element was created after the element’s contents.
‘letter’: The pseudo-element is the first letter of the element.
‘line’: The pseudo-element is the first line of the element.
‘selection’: The selection pseudo-element for the element.

Why are these strings different from the name of the pseudo-element?

The style attribute is a CSSStyleDeclaration [CSSOM] that allows directly setting style information (inline styles) onto the pseudo-element. Inline styles on a CSSPseudoElement have precedence over all style rules styling that pseudo-element.

This should cascade like actual inline styles, not be a different thing.

The EventTarget interface [DOM-LEVEL-2-EVENTS] must be implemented by all instances of CSSPseudoElement as a conformance requirement of this module.

6.2. Interface CSSPseudoElementList

The CSSPseudoElementList represents an ordered collection of CSSPseudoElement instances.

interface CSSPseudoElementList {
    readonly attribute unsigned long length;
    CSSPseudoElement item(unsigned long index);
    CSSPseudoElement getByType(DOMString type);
                     // replies null if no pseudo-element exists for
                     //     the requested type
};

The length attribute represents the number of CSSPseudoElement in the collection or zero if it is empty. The method item() is used to retrieve a CSSPseudoElement by index. It takes one parameter being the requested index into the collection. Its return value is the CSSPseudoElement at the requested index in the collection or null if that is not a valid index.

The method getByType() is used to retrieve a CSSPseudoElement by its type. Its return value is the CSSPseudoElement in the collection that matches the type or null if there is no CSSPseudoElement in the collection for that type.

6.3. Addition to the `window` interface

A new method is added to the Window interface to retrieve pseudo-elements created by a given element for a given type:

partial interface Window {
  CSSPseudoElementList getPseudoElements(Element elt,
                                       DOMString type);
};

The getPseudoElements() method is used to retrieve all CSSPseudoElement instances created by the element elt for the type type. Its return value is a CSSPseudoElementList, potentially empty if no pseudo-element exists for the given element and the given type.

Changes

Changes since the 15 January 2015 Working Draft include:

Addition of the ::marker pseudo-element.
Removed special handling of floated ::first-letter in favor of initial letter styling. [CSS-INLINE-3]
Added ::inactive-selection.
Defined handling of text decorations on ::selection to match IE’s more sensible behavior.
Links to pseudo-element OM design discussions.
Various minor clarifications.

Acknowledgements

The editors would like to thank the following individuals for their contributions, either during the conception of the specification or during its development and specification review process: Tab Atkins, David Baron, Razvan Caliman, Chris Coyier, Anders Grimsrud, Vincent Hardy.

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:

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

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

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

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 [email protected] mailing list.