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, they can be used to 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 [SELECTORS-4].

Note: As a reminder, pseudo-elements cannot be chained together unless explicitly allowed. For example, ::marker::before is not allowed; but ::before::marker is.

2. Typographic Pseudo-elements

2.1. First-Line Text: the ::first-line pseudo-element

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

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

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.

<P>This is a somewhat long HTML paragraph
that will be broken into several lines.
The first line will be styled
by the ‘::first-line’ pseudo-element.
The other lines will be treated
as ordinary lines in the paragraph.</P>

THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be styled by the ‘::first-line’
pseudo-element. The other lines will be 
treated as ordinary lines in the paragraph.

THIS IS A SOMEWHAT LONG
HTML paragraph that will
be broken into several
lines. The first line will
be styled by the
‘::first-line’ pseudo-
element. The other lines
will be treated as ordinary
lines in the paragraph.

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 a block container that establishes an inline formatting context represents the inline-level content of its first line box.

• The first formatted line of a block container or multi-column container that contains block-level content (and is not a table wrapper box) is the first formatted line of its first in-flow block-level child. If no such line exists, it has no first formatted line.

Note: The first formatted line can be an empty line. For example, the first line of the p in <p><br>First… doesn’t contain any letters. Thus the word “First” is not on the first formatted line, and will not be affected by p::first-line.

Note: The first line of a block container that does not itself participate in a block formatting context 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”, but rather the line that contains that entire inline block.

When a first formatted line is represented by multiple ::first-line pseudo-elements, they are nested in the same order as their originating elements. The inline-level contents of this line—​including its root inline box fragment—​are nested within the innermost ::first-line pseudo-element.

<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 box, but with certain restrictions. The following CSS properties apply to a ::first-line pseudo-element:

• all font properties (see [CSS-FONTS-4])
• the color and opacity properties (see [CSS-COLOR-4])
• all background properties (see [CSS-BACKGROUNDS-4])
• any typesetting properties that apply to inline elements (see [CSS-TEXT-3])
• all text decoration properties (see [CSS-TEXT-DECOR-3])
• the ruby-position property (see [CSS-RUBY-1])
• any inline layout properties that apply to inline elements (see [CSS-INLINE-3])
• any other properties defined to apply to ::first-line by their respective specifications

User agents may apply other properties as well except for the following excluded properties:

• writing-mode
• direction
• text-orientation

Note: Setting line-height on ::first-line inherits to the fragment of the root inline box that wraps the contents of the first line, and therefore can both increase and decrease the height of the first line box.

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

During CSS inheritance, the fragment of a child that occurs on the first line inherits any standard inherited properties—​except the properties excluded above—​from the ::first-line pseudo-element. For all other properties, including all custom properties [CSS-VARIABLES-1], inheritance is from the non-pseudo parent. (The portion of a child element that does not occur on the first line always inherits from the non-pseudo parent.)

<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 styled
by the ‘::first-line’ pseudo-element.
The other lines will be treated
as ordinary lines in the paragraph.</p>

<p><p::first-line>This is a somewhat long</p::first-line> HTML paragraph
that will be broken into several lines.
The first line will be styled
by the ‘::first-line’ pseudo-element.
The other lines will be treated
as ordinary lines in the paragraph.</p>

<p><span>This is a somewhat long HTML paragraph
that will be broken into several lines.</span>
The first line will be styled
by the ‘::first-line’ pseudo-element.
The other lines will be treated
as ordinary lines in the paragraph.</p>

<p><p::first-line><span>This is a somewhat long HTML paragraph
that</span></p::first-line><span> will be broken into several lines.</span>
The first line will be styled
by the ‘::first-line’ pseudo-element.
The other lines will be treated
as ordinary lines in the paragraph.</p>

2.2. First-Letter Text: ::first-letter pseudo-element and its ::prefix and ::postfix children

The ::first-letter pseudo-element represents the first Letter, Number, or Symbol (Unicode category L*, N*, or S*) typographic character unit on the first formatted line of its originating element (the first letter) as well as its associated punctuation. Collectively, this text is the first-letter text. The ::first-letter pseudo-element can be used to create “initial caps” and “drop caps”, which are common typographic effects.

h2 + p::first-letter { initial-letter: 3; }

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

To allow independent styling of the first letter itself and its adjacent punctuation, associated preceding punctuation is represented by the ::prefix sub-pseudo-element of the ::first-letter pseudo-element (::first-letter::prefix); and associated following punctuation is represented by the ::postfix sub-pseudo-element of the ::first-letter pseudo-element (::first-letter::postfix). See § 2.2.1 First Letters and Associated Punctuation, below.

2.2.1. First Letters and Associated Punctuation

As explained in CSS Text 3 § 1.4 Characters and Letters, a typographic character 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] When selecting the first letter, the UA should tailor its definition of typographic character unit to reflect the first-letter traditions of the ::first-letter pseudo-element’s containing block’s content language.

Preceding and following punctuation must also be included as part of the first-letter text in the ::first-letter pseudo-element as follows:

• All punctuation—​i.e, characters that belong to the Punctuation (P*) Unicode general category [UAX44]—​that precedes the first letter.
• Any punctuation other than opening punctuation and dashes—​i.e. characters that belong to the Punctuation (P*) Unicode general category, excluding Open Punctuation (Ps) and Dash Punctuation (Pd)—​that follows the first letter.
• Any intervening typographic space—​i.e. characters that belong to the Zs Unicode general category [UAX44] other than U+3000 IDEOGRAPHIC SPACE or any word-separator characters

See CSS Text 3 § 1.4 Characters and Letters and CSS Text 3 § E Characters and Properties for more information on typographic character units and their Unicode properties. [CSS-TEXT-3]

2.2.2. Finding the First-Letter Text

As with ::first-line, the ::first-letter pseudo-element can only have an effect when attached to a block container. Its first-letter text is the first such inline-level content participating in the inline formatting context of its originating element’s first formatted line, if it is not preceded by any other in-flow content (such as images or inline tables) on its line.

For this purpose, any marker boxes are ignored, as if they were out-of-flow. However, if an element has in-flow ::before or ::after content, the first-letter text is selected from the content of the element including that generated content.

If no qualifying text exists, then there is no first-letter text and no ::first-letter pseudo-element.

Note: When the first formatted line is empty, ::first-letter will not match anything. For example, in this HTML fragment: <p><br>First... the first line doesn’t contain any letters, so ::first-letter doesn’t match anything. In particular, it does not match the “F” of “First”, which is on the second line.

Note: As with ::first-line, the first-letter text of a block container that does not participate in a block formatting context cannot be the first-letter text 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.

Any portion of the first-letter text that is wrapped to the next line no longer forms part of the ::first-letter pseudo-element.

2.2.3. Inheritance and Box Tree Structure of the First-Letter Pseudo-elements

The ::first-letter pseudo-element is wrapped immediately around the first-letter text it represents, even if that text is in a descendant. When a first-letter text is represented by multiple ::first-letter pseudo-elements, they are nested in the same order as their originating elements. Inheritance behaves accordingly.

<div>
  <p><span>The first few words</span>
  and the rest of the paragraph.
</div>

<div>
  <p><span><div::first-letter><p::first-letter>T</…></…>he first few words</span>
  and the rest of the paragraph.
</div>

If any ::first-letter::prefix or ::first-letter::postfix pseudo-elements exist, they are nested within the innermost ::first-letter, and otherwise interpreted similar to ::first-letter itself.

<div>
  <p><span>“The first few words</span>
  and the rest of the quotation.
</div>

<div>
  <p><span><div::first-letter><p::first-letter><div::first-letter::prefix><p::first-letter::prefix>“</…></…>T</…></…>he first few words</span>
  and the rest of the paragraph.
</div>

If the characters that would form the first-letter text are not all in the same element (as the ‘T in <p>‘<em>T...), the user agent may create the ::first-letter pseudo-element (and its ::prefix or ::postfix sub-elements, if any) from one of the elements, or all elements, or simply not create the pseudo-element(s). Additionally, if the first-letter text is not at the start of the line (for example due to bidirectional reordering, or due to a list item marker with list-style-position: inside), then the user agent is not required to create the pseudo-element(s).

A ::first-letter pseudo-element is contained within any ::first-line pseudo-elements, and thus inherits (potentially indirectly) from ::first-line, the same as any inline box on the same line.

2.2.4. Styling the First-Letter Pseudo-elements

In CSS a ::first-letter pseudo-element (and its ::prefix and ::postfix sub-elements) is similar to an inline box. The following properties apply to ::first-letter, ::first-letter::prefix, and ::first-letter::postfix pseudo-elements:

• all font properties (see [CSS-FONTS-4])
• the color and opacity properties (see [CSS-COLOR-4])
• all background properties (see [CSS-BACKGROUNDS-4])
• any typesetting properties that apply to inline elements (see [CSS-TEXT-4])
• all text decoration properties (see [CSS-TEXT-DECOR-4])
• any inline layout properties that apply to inline elements (see [CSS-INLINE-3])
• margin and padding properties (see [CSS22])
• border properties and box-shadow (see [CSS-BACKGROUNDS-4])
• any other properties defined to apply to ::first-letter by their respective specifications

User agents may apply other properties as well. However, in no case may the application of such unlisted properties to ::first-letter change what first-letter text is represented by that ::first-letter.

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, to approximate font sizes; and to take the glyph outline into account when performing layout. The possibility of such loosely-defined magic has been intentionally removed, as it proved to be a poor solution for the intended use case (drop caps and raised caps), yet caused interoperability problems. See initial-letter in [CSS-INLINE-3] for explicitly handling drop caps and raised caps.

3. Highlight Pseudo-elements

3.1. Selecting Highlighted Content: the ::selection, ::search-text, ::target-text, ::spelling-error, ::grammar-error, and ::highlight() pseudo-elements

The highlight pseudo-elements represent portions of a document that have been given a particular status and are typically styled differently to indicate that status to the user. For example, selected portions of the document are typically highlighted (given alternate background and foreground colors, or a color wash) to indicate their selected status. The following highlight pseudo-elements are defined:

::selection

The ::selection pseudo-element represents the portion of a document that has been selected as the target or object of some possible future user-agent operation(s). It applies, for example, to selected text within an editable text field, which would be copied by a copy operation or replaced by a paste operation.

Tests

• active-selection-001-manual.html (manual test) (source)
• active-selection-002-manual.html (manual test) (source)
• active-selection-004-manual.html (manual test) (source)
• active-selection-011.html (live test) (source)
• active-selection-012.html (live test) (source)
• active-selection-014.html (live test) (source)
• active-selection-016.html (live test) (source)
• active-selection-018.html (live test) (source)
• active-selection-025.html (live test) (source)
• active-selection-027.html (live test) (source)
• active-selection-056.html (live test) (source)
• active-selection-057.html (live test) (source)
• active-selection-063.html (live test) (source)
• selection-background-color-001.html (live test) (source)
• selection-link-001.html (live test) (source)
• selection-link-002.html (live test) (source)
• selection-link-003.html (live test) (source)
• selection-over-highlight-001.html (live test) (source)
• selection-universal-shadow-dom.html (live test) (source)
• selection-contenteditable-011.html (live test) (source)
• selection-input-011.html (live test) (source)
• selection-textarea-011.html (live test) (source)
• textpath-selection-011.html (live test) (source)

::search-text

The ::search-text pseudo-element represents text identified by the user agent’s find-in-page feature. Since not all UAs style matched text in ways expressible with the highlight pseudo-elements, this pseudo-element is optional to implement.

How to identify the current match? [Issue #10527]

::target-text

The ::target-text pseudo-element represents text directly targeted by the document URL’s fragment, if any.

Note: When a URL fragment targets an element, the :target pseudo-element can be used to select it, but ::target-text does not match anything. It only matches text that is itself targeted by the [fragment].

Tests

• target-text-001.html (live test) (source)
• target-text-002.html (live test) (source)
• target-text-003.html (live test) (source)
• target-text-004.html (live test) (source)
• target-text-005.html (live test) (source)
• target-text-006.html (live test) (source)
• target-text-007.html (live test) (source)
• target-text-008.html (live test) (source)
• target-text-009.html (live test) (source)
• target-text-010.html (live test) (source)
• target-text-dynamic-001.html (live test) (source)
• target-text-dynamic-002.html (live test) (source)
• target-text-dynamic-003.html (live test) (source)
• target-text-dynamic-004.html (live test) (source)
• target-text-shadow-horizontal.html (live test) (source)
• target-text-shadow-vertical.html (live test) (source)
• target-text-text-decoration-001.html (live test) (source)

::spelling-error

The ::spelling-error pseudo-element represents a portion of text that has been flagged by the user agent as misspelled.

Tests

• spelling-error-001.html (live test) (source)
• spelling-error-002-manual.html (manual test) (source)
• spelling-error-003-manual.html (manual test) (source)
• spelling-error-004-crash.html (live test) (source)
• spelling-error-005-crash.html (live test) (source)
• spelling-error-006.html (live test) (source)

::grammar-error

The ::grammar-error pseudo-element represents a portion of text that has been flagged by the user agent as grammatically incorrect.

Tests

• grammar-error-001.html (live test) (source)
• grammar-error-002-manual.html (manual test) (source)
• grammar-error-003-manual.html (manual test) (source)

::highlight(<custom-ident>)

The ::highlight() functional pseudo-element represents the portion of the document associated with the custom highlight identified by the given custom highlight name. The <custom-ident> argument is required. See [CSS-HIGHLIGHT-API-1] for details.

Tests

The highlight pseudo-elements do not necessarily fit into the element tree, and can arbitrarily cross element boundaries without honoring its nesting structure.

3.2. Styling Highlights

The highlight pseudo-elements can only be styled by a limited set of properties that do not affect layout and can be applied performantly in a highly dynamic environment—​and additionally (to ensure interoperability) whose rendering within the required area is not dependent on the exact (UA-determined) bounds of the highlight overlay.

The following properties apply to the highlight pseudo-elements:

• color
  Tests

  • active-selection-001-manual.html (manual test) (source)
  • active-selection-011.html (live test) (source)
  • active-selection-016.html (live test) (source)
  • active-selection-018.html (live test) (source)
• background-color
  Tests

  • active-selection-002-manual.html (manual test) (source)
  • active-selection-012.html (live test) (source)
  • active-selection-031.html (live test) (source)
• text-decoration and its associated properties (including text-underline-position and text-underline-offset)
  Tests

  • active-selection-004-manual.html (manual test) (source)
  • active-selection-014.html (live test) (source)
  • active-selection-021.html (live test) (source)
  • grammar-error-001.html (live test) (source)
  • grammar-error-002-manual.html (manual test) (source)
  • grammar-error-003-manual.html (manual test) (source)
  • spelling-error-001.html (live test) (source)
  • spelling-error-002-manual.html (manual test) (source)
  • spelling-error-003-manual.html (manual test) (source)
• text-shadow
  Tests

  • marker-text-shadow.html (live test) (source)
• stroke-color, fill-color, and stroke-width
  Tests

  • textpath-selection-011.html (live test) (source)

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.

Note: The color property sets the color of both the text and all line decorations (underline, overline, line-through) and emphasis marks (text-emphasis) applied to the text by the originating element and its ancestors and descendants.

For any properties not listed above, but which are required to resolve the values of applicable properties, their computed values are copied from those of the originating element, ignoring any values specified on the highlight pseudo-element itself. For example:

• forced-color-adjust (used in forced colors mode to resolve colors) and color-scheme (used to resolve system colors)
• font-size, font-family, etc. (used to resolve em and other font-relative lengths).
• line-height (used to resolve lh units)
• custom properties [CSS-VARIABLES-1] (used in var() substitutions)

Should color-scheme go here, or in the list of applicable properties? [Issue #11011]

If, for a given highlight pseudo-element, there are colors specified in the author origin, those colors must be respected as specified; i.e. the UA must not apply any extra processing (such as using semi-transparent washes). However if there are no colors in the author origin, the UA may apply additional color processing.

Note: This is to ensure that color contrast is preserved across all user agents interpreting a given author stylesheet.

Vendor-prefixed properties such as -webkit-text-fill-color are not applicable to the highlight pseudo-elements.

3.3. Default UA Styles

The following additions are recommended for the default UA stylesheet:

/* Represent default spelling/grammar error styling in an adjustable way */
:root::spelling-error { text-decoration: spelling-error; }
:root::grammar-error  { text-decoration: grammar-error; }

Some highlight pseudo-elements should have paired default highlight colors—​a default color and background-color provided by the UA that are either used or overridden together, see § 3.3.1 Paired Defaults. For ::selection they should correspond to HighlightText and Highlight, while for ::target-text they should correspond to MarkText and Mark.

UAs may apply additional effects to enhance the presentation of highlighted content, for example dimming content other than the highlighted text or transitioning out a highlight style based on user interactions or timing. These are not controlled by CSS.

UA tweaks to the presentation of highlights in ways that are controlled by CSS are currently under discussion in Issue 6853.

3.3.1. Paired Defaults

For compatibility reasons, paired default highlight colors must only be used when neither color nor background-color yield a cascaded value from the author origin (or inherit their value from the author origin). When a highlight color is revert or revert-layer, the origin after rolling back the cascade determines the cascaded value’s origin.

Note: Because this rule is for compatibility reasons, it does not apply to other similar properties like fill-color or stroke-color.

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

em::selection { color: initial; }
em::selection { color: inherit; }
em::selection { color: unset; }
em::selection { color: green; }
p::selection { color: green; }

3.4. Area of a Highlight

For each type of highlighting (see § 3.1 Selecting Highlighted Content: the ::selection, ::search-text, ::target-text, ::spelling-error, ::grammar-error, and ::highlight() 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.
  Tests

  • selection-intercharacter-011.html (live test) (source)
  • selection-intercharacter-012.html (live test) (source)
• 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.
  Tests

  • active-selection-043.html (live test) (source)
• The overlay may also include other areas within the border-box of an element; in this case, those areas belong to the innermost such element that contains the area.
• For an inline-level box, the overlay may extend outside its border edges in the block axis as far as the edges of its line box.

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.

3.5. Cascading and Per-Element Highlight Styles

Each element draws its own active portions of the highlight overlays, which receives the styles specified by the corresponding highlight pseudo-element styles for which that element is the originating element. When multiple styles conflict, the winning style is determined by the cascade.

When any supported property is not given a value by the cascade, or given a value of inherit or unset, its specified value is determined by inheritance from the corresponding highlight pseudo-element of its originating element’s parent element. This occurs regardless of whether that property is an inherited property (and regardless of whether that property is a custom property that is registered to inherit or not).

Additionally, for highlight pseudo-elements originating from the root element:

• the inherited value of color is currentColor, not the initial value.
• custom properties inherit from the root element.

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

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

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.

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

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

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

The following example demonstrates the inheritance of custom properties.

:root {
   --background-color: green;
   --decoration-thickness: 10px;
   --decoration-color: purple;
 }
 ::selection {
   --decoration-thickness: 1px;
   --decoration-color: green;
 }
 div::selection {
   --decoration-color: blue;
   background-color: var(--background-color, red);
   text-decoration-line: underline;
   text-decoration-style: line;
   text-decoration-thickness: var(--decoration-thickness, 5px);
   text-decoration-color: var(--decoration-color, red);
 }

Note: This behavior is intended to accomodate the standard practice of defining document-wide custom properties on :root.

3.6. Painting the Highlight

3.6.1. Backgrounds

Each highlight pseudo-element draws its background over the corresponding portion of the highlight overlay, painting it immediately below any positioned descendants (i.e. just before step 8 in CSS2.2§E.2). The ::selection overlay is drawn over the ::target-text overlay which is drawn over the ::spelling-error overlay which is drawn over the ::grammar-error overlay which is drawn over the ::highlight overlays. The ::search-text overlay is drawn directly over or below the :selection overlay depending on the UA, and drawn over all other overlays.

3.6.2. Shadows

Any text-shadow applying to a highlight pseudo-element is drawn over its corresponding highlight overlay background. Such text shadows also stack over each other (and over any original text-shadow applied to the text and its decorations, which continues to apply).

Note: Since each highlight overlay background is drawn over any shadows belonging to the layer(s) below, a highlight overlay background can obscure lower-level shadows.

3.6.3. Text and Text Decorations

A highlight pseudo-element suppresses the normal drawing of any associated text, and the text decorations (other than shadows) that had been applied to that text. Instead the topmost active highlight overlay redraws that text (and those decorations) over all the highlight overlay backgrounds using that highlight’s own color.

Note: This means that unlike shadows, line decorations and emphasis marks won’t be obscured by any highlight overlay backgrounds that are drawn for the associated text.

For this purpose, currentColor on a highlight pseudo-element’s color property represents the color of the next active highlight pseudo-element layer below, falling back finally to the colors that would otherwise have been used (those applied by the originating element or an intervening pseudo-element such as ::first-line or ::first-letter).

Note: The element’s own text decorations (both line decorations and emphasis marks) are thus drawn in the pseudo-element’s own color when that is not currentColor, regardless of their original color or fill specifications.

Any text decorations introduced by each highlight pseudo-element are stacked in the same order as their backgrounds over the text’s original decorations and are all drawn, each decoration in its own color. The normal painting order applies, so as per CSS2 Appendix E, all underlines are drawn below overlines which are drawn below the text which is drawn below any line-throughs. However, text decorations applied by ::selection may instead all be drawn along with the text as a topmost set of layers above all other decorations.

Line decorations introduced by highlight pseudo-elements apply only to the text associated with their originating element, and are not propagated to descendants except via property inheritance (as described above).

Note: Unlike the originating element’s own decorations, decorations declared on a highlight pseudo-element propagate to out-of-flow elements and inline blocks, with thickness and position varying between descendants.

3.6.4. Replaced Elements

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.

3.7. Security Considerations for Highlighting

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 their 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

Tree-abiding pseudo-elements always fit within the box tree. They inherit any inheritable properties from their originating element; non-inheritable properties take their initial values as usual. [CSS-CASCADE-4] A subset of these are the fully styleable pseudo-elements; all properties that apply to a real element also apply to a fully styleable pseudo-element.

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, with content as specified by content.

::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.

Both ::before and ::after are fully styleable pseudo-elements: there is no restriction on what properties apply to them.

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

As with the content of regular elements, the generated content of ::before and :after pseudo-elements can form part of any ::first-line and ::first-letter pseudo-elements applied to its originating element. Also as with regular child elements, the ::before and ::after pseudo-elements are suppressed when their parent, the originating element, is replaced.

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

The ::marker pseudo-element represents the automatically generated marker box of a list item. (See [CSS-DISPLAY-3] and [CSS-LISTS-3].)

The contents of a ::marker are ignored (not selected) by ::first-letter.

Interaction of ::marker and ::first-line is currently under discussion in Issue 4506.

::marker is a tree-abiding pseudo-element, but is not fully styleable; only a limited set of properties can be used on the ::marker pseudo-element. This list is defined in CSS Lists 3 § 3.1.1 Properties Applying to ::marker.

The ::before::marker or ::after::marker selectors are valid and can be used to represent the marker boxes of ::before or ::after pseudo-elements that happen to be list items. However ::marker::marker is invalid, and the computed value of display on ::marker always loses any list-item aspect.

Should ::marker also have ::prefix and ::postfix sub-elements?

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. It is a tree-abiding pseudo-element.

Note: There also exists a :placeholder-shown pseudo-class, which applies to (real) elements while they are showing placeholder text, and can be used to style such elements specially. ::placeholder specifically represents the placeholder text, and is thus relatively limited in its abilities.

All properties that apply to the ::first-line pseudo-element also apply to the ::placeholder pseudo-element, except those defined in [CSS-INLINE-3].

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.

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

Note: It’s been requested that ::placeholder also refer to a placeholder which has a corresponding element in the element tree. It’s not clear how this should work, but it may be worth doing. See Issue 2417.

5. Element-backed Pseudo-Elements

The element-backed pseudo-elements, interact with most CSS and other platform features as if they were real elements (and, in fact, often are real elements that are not otherwise selectable).

Unless otherwise specified, they are fully styleable and inherit from their originating element, just like standard tree-abiding pseudo-elements; but they can be defined to inherit from another element instead. (For example, ::part() inherits from the parent of the element it represents in the shadow tree.)

All pseudo-classes and pseudo-elements are syntactically allowed after an element-backed pseudo-element (such as x-button::part(label):hover or x-button::part(label)::before), just as if the pseudo-element were a type selector; but some are disallowed from matching:

• The structural pseudo-classes, :has() pseudo-class, :scope pseudo-class, and :host/:host()/:host-context() pseudo-classes never match.

• ::part() never matches. (Other element-backed pseudo-elements can, however.)

An element-backed pseudo-element can define itself as representing a real element (possibly one not accessible in the current tree). If it does so, all pseudo-classes and pseudo-elements not otherwise disallowed (see above) match as they would on that real element. If it does not do so, it must define which pseudo-classes it matches and when; however, unless otherwise specified, any pseudo-classes allowed on tree-abiding pseudo-elements are always allowed on element-backed pseudo-elements.

5.1. File Selector Button: the ::file-selector-button pseudo-element

The ::file-selector-button pseudo-element targets the <button> inside an <input> element with type=file, if the UA renders such a button. It is an element-backed pseudo-element.

There is no restriction on which properties apply to the ::file-selector-button pseudo-element.

::file-selector-button { border: 3px solid green }

5.2. Expandable contents of details element: the ::details-content pseudo-element

The ::details-content pseudo-element targets the additional information in a a details element that can be expanded or collapsed. It is an element-backed pseudo-element.

There is no restriction on which properties apply to the ::details-content pseudo-element.

details::details-content {
  opacity: 0;
  transition: content-visibility 300ms allow-discrete, opacity 300ms;
}

details[open]::details-content {
  opacity: 1;
}

6. Overlapping Pseudo-element Interactions

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

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

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

7. Additions to the CSS Object Model

7.1. CSSPseudoElement Interface

The CSSPseudoElement interface allows pseudo-elements to be event targets.

[Exposed=Window]
interface CSSPseudoElement : EventTarget {
    readonly attribute CSSOMString type;
    readonly attribute Element element;
    readonly attribute (Element or CSSPseudoElement) parent;
    CSSPseudoElement? pseudo(CSSOMString type);
};

This interface is under design development, and this draft is looking for feedback more than implementation. The CSSWG would particularly appreciate hearing about use cases and problems.

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

"::before"

Represents the ::before pseudo-element.

"::after"

Represents the ::after pseudo-element.

"::marker"

Represents the ::marker pseudo-element.

The element attribute is the ultimate originating element of the pseudo-element.

The parent attribute is the originating element of the pseudo-element. For most pseudo-elements parent and element will return the same Element; for sub-pseudo-elements, parent will return a CSSPseudoElement while element returns an Element.

The pseudo(type) method returns the CSSPseudoElement interface representing the sub-pseudo-element referenced in its argument, if such a sub-pseudo-element could exist and would be valid, and null otherwise. See pseudo() below.

Note: This interface may be extended in the future to other pseudo-element types and/or to allow setting style information through a CSSStyleDeclaration style attribute. The current functionality is limited to that which is needed to support [web-animations-1].

7.2. pseudo() method of the Element interface

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

partial interface Element {
  CSSPseudoElement? pseudo(CSSOMString type);
};

Return values that represent the same pseudo-element on the same originating element must be, insofar as observable, always the same CSSPseudoElement object. (The UA may drop or regenerate the object for convenience or performance if this is not observable.)

The identity, lifetime, and nullness of the return value (and potential error cases) of the pseudo() method is still under discussion. See Issue 3607 and Issue 3603.

7.3. getComputedStyle()

When the second parameter pseudoElt refers to a highlight pseudo-element, getComputedStyle() returns styles as if that highlight is active and all other highlights are inactive. This avoids the potential ambiguity and privacy risks of returning a result that depends on the actual highlight state.

8. Compatibility Syntax

For compatibility with existing style sheets written against CSS Level 2 [CSS2], user agents must also accept the previous one-colon notation (:before, :after, :first-letter, :first-line) for the ::before, ::after, ::first-letter, and ::first-line pseudo-elements.

Security Considerations

For highlighting, see § 3.7 Security Considerations for Highlighting.

Privacy Considerations

No new privacy considerations have been raised against this module.

Changes

Significant changes since the 30 December 2022 Working Draft include:

• Defined element-backed pseudo-element. (Issue 10199, Issue 10083)
• Added ::details-content pseudo-element (Issue 10379)
• Clarified that the inherited value of color is currentColor, not the initial value.
• Clarified that a custom property may be defined and/or used inside a highlight pseudo-element.
• Defined the behavior when custom property values are not found in the highlight cascade; they are taken from the root. (Issue 6641)

Significant changes since the 31 December 2020 Working Draft include:

• Added ::first-letter::prefix and ::first-letter::postfix sub-pseudo-elements to address punctuation associated with the ::first-letter pseudo-element. (Issue 2040)
• Fine-tuned the first-letter text pattern:
  • Excluded word-separating spaces from first-letter text. (Issue 5830)
  • Excluded dashes and opening punctuation from following punctuation. (Issue 5830)
  • Allowed symbols to represent the first letter. (Issue 5099)
  • Clarified that the first-letter text is truncated by a line break. (Issue 2254)
• Rewrote and clarified definition of :first-letter, and improved examples and notes.
• Rewrote and clarified definition of ::first-line, and improved examples.
• Defined application of line-height to ::first-line. (Issue 2282)
• Defined box structure interaction of ::first-line and ::first-letter.
• Defined hierarchy order of ::first-line and the root inline box. (Issue 1384)
• Clarified paired default behavior of highlight pseudo-element color and background-color. (Issue 6386, Issue 6774, Issue 6779, Issue 7837)
• Defined application of forced colors mode to highlight pseudo-elements. (Issue 7264)
• Clarified painting order, propagation, and application of highlight pseudo-element text decorations. (Issue 6022, Issue 6829, Issue 7101)
• Removed caret-color and cursor from applying to highlight pseudo-elements. (Issue 4100)
• Clarified interaction of highlight pseudo-elements and getComputedStyle(). (Issue 6818)
• Clarified application of custom properties to pseudo-elements. (Issue 6264)
• Extended the CSSPseudoElement interface with a parent attribute and pseudo() method to address sub-pseudo-elements. (Issue 3836)

Significant changes since the 25 February 2019 Working Draft include:

• Added ::target-text pseudo-element. (Issue 5522)
• Removed ::inactive-selection. (Issue 4579)
• Added ::file-selectors-button pseudo-element. (Issue 5049)
• Adjusted rules for inheritance of properties from ::first-line to handle inheritable vs non-inheritable properties differently, specifically excluded applicability of properties that control writing mode because they affect the cascade, and defined interaction with custom properties. (Issue 1097)
• Added ruby-position to properties allowed on ::first-line, analogous with text-emphasis-position. (Issue 2998)
• Included spaces between the first letter and its surrounding punctuation in ::first-letter. (Issue 5154)
• Defined interaction of ::marker and ::first-letter.
• Made ::before::marker and ::after::marker valid. (Issue 1793)
• Moved the list of properties applying to ::marker to [CSS-LISTS-3] (also adding content, unicode-bidi, and direction).
• Added ::marker to types allowed for CSSPseudoElement interface. (Issue 3763)
• Clarified return value of pseudo(). (Issues 3603 and 3607)
• Allowed highlight overlay of inline-level boxes to extend to the line box edges. (Issues 5395 and 4624)
• Clarified that currentColor on a highlight pseudo-element uses the color that would be used if the text were not “highlighted”, even if that color was provided by a pseudo-element like ::first-line. (Issue 4625)
• Defined how text-shadow interacts with highlight pseudo-elements. (Issue 3932)
• Improved the default UA style sheet rules applying to highlight pseudo-elements.

Changes since the 7 June 2016 Working Draft include:

• Specified spelling-error and grammar-error with ::spelling-error and ::grammar-error in the UA stylesheet.
• Redefined value propagation between parent/child highlight pseudo-elements to use inheritance rather than cascading. See discussion in and linked from Issue 2474.
• Refined list of supported properties for highlight pseudo-elements, e.g. adding stroke-color/fill-color, removing outline, etc.
• Clarified how text and text decoration colors are painted for highlight pseudo-elements.
• Added the element attribute to the CSSPseudoElement interface.
• Changed the values of the type attribute on the CSSPseudoElement interface to match the corresponding pseudo-elements.
• Reduced supported types of CSSPseudoElement and removed unimplemented style attribute.
• Changed window.getPseudoElements(elem,type) to Element.pseudo(type).
• Miscellaneous minor clarifications and fixes.

Acknowledgements

The editors would like to specifically thank the following individuals for their contributions to this specification: Tab Atkins, David Baron, Oriol Brufau, Razvan Caliman, Chris Coyier, Anders Grimsrud, Vincent Hardy, François Remy.