CSS Cascading and Inheritance Level 5

W3C Working Draft,

This version:
Latest published version:
Editor's Draft:
Previous Versions:
Issue Tracking:
CSSWG Issues Repository
Inline In Spec
Elika J. Etemad / fantasai (Invited Expert)
Miriam E. Suzanne (Invited Expert)
Tab Atkins Jr. (Google)
Suggest an Edit for this Spec:
GitHub Editor


This CSS module describes how to collate style rules and assign values to all properties on all elements. By way of cascading and inheritance, values are propagated for all properties on all elements.

New in this level is cascade layers.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, 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 https://www.w3.org/TR/.

This document was published by the CSS Working Group as a Working Draft. 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.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-cascade” in the title, like this: “[css-cascade] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 15 September 2020 W3C Process Document.

This document was produced by a group operating under the 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.

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

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

1. Introduction

CSS defines a finite set of parameters, called properties, that direct the rendering of a document. Each property has a name (e.g., color, font-size, or border-style), a value space (e.g., <color>, <length-percentage>, [ solid | dashed | dotted | … ]), and a defined behavior on the rendering of the document. Properties values are assigned to various parts of the document via property declarations, which assign the property a value (e.g. red, 12pt, dotted) for the associated element or box.

One of the fundamental design principles of CSS is cascading, which allows several style sheets to influence the presentation of a document. When different declarations try to set a value for the same element/property combination, the conflicts must somehow be resolved.

The opposite problem arises when no declarations try to set a the value for an element/property combination. In this case, a value is be found by way of inheritance or by looking at the property’s initial value.

The cascading and defaulting process takes a set of declarations as input, and outputs a specified value for each property on each element.

The rules for finding the specified value for all properties on all elements in the document are described in this specification. The rules for finding the specified values in the page context and its margin boxes are described in [css-page-3].

1.1. Module Interactions

This section is normative.

This module replaces and extends the rules for assigning property values, cascading, and inheritance defined in [CSS2] chapter 6.

Other CSS modules may expand the definitions of some of the syntax and features defined here. For example, the Media Queries Level 4 specification, when combined with this module, expands the definition of the <media-query> value type as used in this specification.

For the purpose of this specification, text nodes are treated as element children of their associated element, and possess the full set of properties; since they cannot be targeted by selectors all of their computed values are assigned by defaulting.

2. Importing Style Sheets: the @import rule

The @import rule allows users to import style rules from other style sheets. If an @import rule refers to a valid stylesheet, user agents must treat the contents of the stylesheet as if they were written in place of the @import rule, with two exceptions:

For example, declarations in style rules from imported stylesheets interact with the cascade as if they were written literally into the stylesheet at the point of the @import.

Any @import rules must precede all other valid at-rules and style rules in a style sheet (ignoring @charset and empty @layer definitions), or else the @import rule is invalid. The syntax of @import is:

@import [ <url> | <string> ]
        [ layer | layer(<layer-ident>) ]?
        [ supports( [ <supports-condition> | <declaration> ] ) ]?
        <media-query-list>? ;


The following conditional @import rule only loads the style sheet when the UA supports display: flex, and only applies the style sheet on a handheld device with a maximum viewport width of 400px.
@import url("narrow.css") supports(display: flex) handheld and (max-width: 400px);
The following layer imports load the style sheets into the framework component layer, and an un-named layer, respectively:
@import url("tabs.css") layer(framework component);
@import url("override.css") layer();

If a <string> is provided, it must be interpreted as a <url> with the same value.

The following lines are equivalent in meaning and illustrate both @import syntaxes (one with url() and one with a bare string):
@import "mystyle.css";
@import url("mystyle.css");

2.1. Conditional @import Rules

The import conditions allow the import to be media– or feature-support–dependent. In the absence of any import conditions, the import is unconditional. (Specifying all for the <media-query-list> has the same effect.) If the import conditions do not match, the rules in the imported stylesheet do not apply, exactly as if the imported stylesheet were wrapped in @media and/or @supports blocks with the given conditions.

The following rules illustrate how @import rules can be made media-dependent:
@import url("fineprint.css") print;
@import url("bluish.css") projection, tv;
@import url("narrow.css") handheld and (max-width: 400px);

User agents may therefore avoid fetching a conditional import as long as the import conditions do not match. Additionally, if a <supports-condition> blocks the application of the imported style sheet, the UA must not fetch the style sheet (unless it is loaded through some other link) and must return null for the import rule’s CSSImportRule.styleSheet value (even if it is loaded through some other link).

The following rule illustrates how an author can provide fallback rules for legacy user agents without impacting network performance on newer user agents:
@import url("fallback-layout.css") supports(not (display: flex));
@supports (display: flex) {

A <media-query> corresponds to the media_query_list production and is interpreted as a media query, and a <supports-condition> corresponds to a supports_condition production and is interpreted as an @supports condition. If a <declaration> (a declaration production) is given in place of a <supports-condition>, it must be interpreted as a supports_declaration_condition production (i.e. the extra set of parentheses is implied) and treated as a <supports-condition>.

For example, the following two lines are equivalent:
@import "mystyle.css" supports(display: flex);
@import "mystyle.css" supports((display: flex));

The evaluation and full syntax of the import conditions are defined by the Media Queries [MEDIAQ] and CSS Conditional Rules [CSS-CONDITIONAL-3] specifications.

2.2. Processing Stylesheet Imports

When the same style sheet is imported or linked to a document in multiple places, user agents must process (or act as though they do) each link as though the link were to an independent style sheet.

Note: This does not place any requirements on resource fetching, only how the style sheet is reflected in the CSSOM and used in specs such as this one. Assuming appropriate caching, it is perfectly appropriate for a UA to fetch a style sheet only once, even though it’s linked or imported multiple times.

The origin of an imported style sheet is the origin of the style sheet that imported it.

The environment encoding of an imported style sheet is the encoding of the style sheet that imported it. [css-syntax-3]

2.3. Content-Type of CSS Style Sheets

The processing of imported style sheets depends on the actual type of the linked resource:

If the linked resource’s type is text/css, it must be interpreted as a CSS style sheet. Otherwise, it must be interpreted as a network error.

3. Shorthand Properties

Some properties are shorthand properties, meaning that they allow authors to specify the values of several properties with a single property. A shorthand property sets all of its longhand sub-properties, exactly as if expanded in place.

When values are omitted from a shorthand form, unless otherwise defined, each “missing” sub-property is assigned its initial value.

This means that a shorthand property declaration always sets all of its sub-properties, even those that are not explicitly set. Carelessly used, this might result in inadvertently resetting some sub-properties. Carefully used, a shorthand can guarantee a “blank slate” by resetting sub-properties inadvertently cascaded from other sources.

For example, writing background: green rather than background-color: green ensures that the background color overrides any earlier declarations that might have set the background to an image with background-image.

For example, the CSS Level 1 font property is a shorthand property for setting font-style, font-variant, font-weight, font-size, line-height, and font-family all at once. The multiple declarations of this example:
h1 {
  font-weight: bold;
  font-size: 12pt;
  line-height: 14pt;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal;

can therefore be rewritten as

h1 { font: bold 12pt/14pt Helvetica }

As more font sub-properties are introduced into CSS, the shorthand declaration resets those to their initial values as well.

In some cases, a shorthand might have different syntax or special keywords that don’t directly correspond to values of its sub-properties. (In such cases, the shorthand will explicitly define the expansion of its values.)

In other cases, a property might be a reset-only sub-property of the shorthand: Like other sub-properties, it is reset to its initial value by the shorthand when unspecified, but the shorthand might not include syntax to set the sub-property to any of its other values. For example, the border shorthand resets border-image to its initial value of none, but has no syntax to set it to anything else. [css-backgrounds-3]

If a shorthand is specified as one of the CSS-wide keywords [css-values-3], it sets all of its sub-properties to that keyword, including any that are reset-only sub-properties. (Note that these keywords cannot be combined with other values in a single declaration, not even in a shorthand.)

Declaring a shorthand property to be !important is equivalent to declaring all of its sub-properties to be !important.

3.1. Aliasing

Properties sometimes change names after being supported for a while, such as vendor-prefixed properties being standardized. The original name still needs to be supported for compatibility reasons, but the new name is preferred. To accomplish this, CSS defines two different ways of “aliasing” old syntax to new syntax.

legacy name aliases
When the old property’s value syntax is identical to that of the new property, the two names are aliased with an operation on par with case-mapping: at parse time, the old property is converted into the new property. This conversion also applies in the CSSOM, both for string arguments and property accessors: requests for the old property name transparently transfer to the new property name instead.
For example, if old-name is a legacy name alias for new-name, getComputedStyle(el).oldName will return the computed style of the newName property, and el.style.setPropertyValue("old-name", "value") will set the new-name property to "value".
legacy shorthands
When the old property has a distinct syntax from the new property, the two names are aliased using the shorthand mechanism. These shorthands are defined to be legacy shorthands, and their use is deprecated. They otherwise behave exactly as regular shorthands, except that the CSSOM will not use them when serializing declarations. [CSSOM]
For example, the page-break-* properties are legacy shorthands for the break-* properties (see CSS Fragmentation 3 §3.4 Page Break Aliases: the page-break-before, page-break-after, and page-break-inside properties).

Setting page-break-before: always expands to break-before: page at parse time, like other shorthands do. Similarly, if break-before: page is set, calling getComputedStyle(el).pageBreakBefore will return "always". However, when serializing a style block (see CSSOM 1 §5.7.2 Serializing CSS Values), the page-break-before property will never be chosen as the shorthand to serialize to, regardless of whether it or break-before was specified; instead, break-before will always be chosen.

3.2. Resetting All Properties: the all property

Name: all
Value: initial | inherit | unset | revert | revert-layer
Initial: see individual properties
Applies to: see individual properties
Inherited: see individual properties
Percentages: see individual properties
Computed value: see individual properties
Animation type: see individual properties
Canonical order: per grammar

The all property is a shorthand that resets all CSS properties except direction and unicode-bidi. It only accepts the CSS-wide keywords. It does not reset custom properties [css-variables-1].

Note: The excepted CSS properties direction and unicode-bidi are actually markup-level features, and should not be set in the author’s style sheet. (They exist as CSS properties only to style document languages not supported by the UA.) Authors should use the appropriate markup, such as HTML’s dir attribute, instead. [css-writing-modes-3]

For example, if an author specifies all: initial on an element, it will block all inheritance and reset all properties, as if no rules appeared in the author, user, or user-agent levels of the cascade.

This can be useful for the root element of a "widget" included in a page, which does not wish to inherit the styles of the outer page. Note, however, that any "default" style applied to that element (such as, e.g. display: block from the UA style sheet on block elements such as <div>) will also be blown away.

4. Value Processing

Once a user agent has parsed a document and constructed a document tree, it must assign, to every element in the tree, and correspondingly to every box in the formatting structure, a value to every property that applies to the target media type.

The final value of a CSS property for a given element or box is the result of a multi-step calculation:

  1. First, all the declared values applied to an element are collected, for each property on each element. There may be zero or many declared values applied to the element.
  2. Cascading yields the cascaded value. There is at most one cascaded value per property per element.
  3. Defaulting yields the specified value. Every element has exactly one specified value per property.
  4. Resolving value dependencies yields the computed value. Every element has exactly one computed value per property.
  5. Formatting the document yields the used value. An element only has a used value for a given property if that property applies to the element.
  6. Finally, the used value is transformed to the actual value based on constraints of the display environment. As with the used value, there may or may not be an actual value for a given property on an element.

Elements that are not connected or are not part of the document’s flattened element tree do not participate in CSS value processing, and do not have declared, cascaded, specified, computed, used, or actual values, even if they potentially have style declarations assigned to them (for example, by a style attribute).

4.1. Declared Values

Each property declaration applied to an element contributes a declared value for that property associated with the element. See Filtering Declarations for details.

These values are then processed by the cascade to choose a single “winning value”.

4.2. Cascaded Values

The cascaded value represents the result of the cascade: it is the declared value that wins the cascade (is sorted first in the output of the cascade). If the output of the cascade is an empty list, there is no cascaded value.

4.3. Specified Values

The specified value is the value of a given property that the style sheet authors intended for that element. It is the result of putting the cascaded value through the defaulting processes, guaranteeing that a specified value exists for every property on every element.

In many cases, the specified value is the cascaded value. However, if there is no cascaded value at all, the specified value is defaulted. The CSS-wide keywords are handled specially when they are the cascaded value of a property, setting the specified value as required by that keyword, see § 7.3 Explicit Defaulting.

4.4. Computed Values

The computed value is the result of resolving the specified value as defined in the “Computed Value” line of the property definition table, generally absolutizing it in preparation for inheritance.

Note: The computed value is the value that is transferred from parent to child during inheritance. For historical reasons, it is not necessarily the value returned by the getComputedStyle() function, which sometimes returns used values. [CSSOM] Furthermore, the computed value is an abstract data representation: their definitions reflect that data representation, not how that data is serialized. For example, serialization rules often allow omitting certain values which are implied during parsing; but those values are nonetheless part of the computed value.

A specified value can be either absolute (i.e., not relative to another value, as in red or 2mm) or relative (i.e., relative to another value, as in auto, 2em). Computing a relative value generally absolutizes it:

See examples (f), (g) and (h) in the table below.

Note: In general, the computed value resolves the specified value as far as possible without laying out the document or performing other expensive or hard-to-parallelize operations, such as resolving network requests or retrieving values other than from the element and its parent.

The computed value exists even when the property does not apply. However, some properties may change how they determine the computed value based on whether the property applies to the element.

4.5. Used Values

The used value is the result of taking the computed value and completing any remaining calculations to make it the absolute theoretical value used in the formatting of the document.

For example, a declaration of width: auto can’t be resolved into a length without knowing the layout of the element’s ancestors, so the computed value is auto, while the used value is an absolute length, such as 100px. [CSS2]

As another example, a <div> might have a computed break-before value of auto, but acquire a used break-before value of page by propagation from its first child. [css-break-3]

If a property does not apply to this element or box type then it has no used value for that property.

For example, the flex property has no used value on elements that aren’t flex items.

Note: A property defined to apply to “all elements” applies to all elements and display types, but not necessarily to all pseudo-element types, since pseudo-elements often have their own specific rendering models or other restrictions. The ::before and ::after pseudo-elements, however, are defined to generate boxes almost exactly like normal elements and are therefore defined accept all properties that apply to “all elements”. See [CSS-PSEUDO-4] for more information about pseudo-elements.

4.5.1. Applicable Properties

If a property does not apply to an element or box type—as noted in its “Applies to” line—this means it does not directly take effect on that type of box or element.

Note: A property that does not apply can still have indirect formatting effects if its computed value affects the computation of other properties that do apply; and of course its computed value, which always exists, can still inherit to descendants and take effect on them.

Even though writing-mode and text-orientation do not apply to table rows (they do not affect how the table row or its children are laid out), setting them on such boxes will still affect the calculation of font relative units such as ch, and thus possibly any property that takes a <length>.
Setting text-transform on an HTML p element (which is display: block by default) will have an effect, even though text-transform only applies to inline boxes, because the property inherits into the paragraph’s anonymous root inline box and applies to the text it contains.

Note: A property defined to apply to “all elements” applies to all elements and display types, but not necessarily to all pseudo-element types, since pseudo-elements often have their own specific rendering models or other restrictions. The ::before and ::after pseudo-elements, however, are defined to generate boxes almost exactly like normal elements and are therefore defined accept all properties that apply to “all elements”. See [CSS-PSEUDO-4] for more information about pseudo-elements.

4.6. Actual Values

A used value is in principle ready to be used, but a user agent may not be able to make use of the value in a given environment. For example, a user agent may only be able to render borders with integer pixel widths and may therefore have to approximate the used width. Also, the font size of an element may need adjustment based on the availability of fonts or the value of the font-size-adjust property. The actual value is the used value after any such adjustments have been made.

Note: By probing the actual values of elements, much can be learned about how the document is laid out. However, not all information is recorded in the actual values. For example, the actual value of the page-break-after property does not reflect whether there is a page break or not after the element. Similarly, the actual value of orphans does not reflect how many orphan lines there is in a certain element. See examples (j) and (k) in the table below.

4.7. Examples

Examples of CSS Value Computation
Property Winning declaration Cascaded value Specified value Computed value Used value Actual value
(a) text-align text-align: left left left left left left
(b) border-top-width, border-right-width, border-bottom-width, border-left-width border-width: inherit inherit 4.2px 4.2px 4.2px 4px
(c) width (none) (none) auto (initial value) auto 120px 120px
(d) list-style-position list-style-position: inherit inherit inside inside inside inside
(e) list-style-position list-style-position: initial initial outside (initial value) outside outside outside
(f) font-size font-size: 1.2em 1.2em 1.2em 14.1px 14.1px 14px
(g) width width: 80% 80% 80% 80% 354.2px 354px
(h) width width: auto auto auto auto 134px 134px
(i) height height: auto auto auto auto 176px 176px
(j) page-break-after (none) (none) auto (initial value) auto auto auto
(k) orphans orphans: 3 3 3 3 3 3

5. Filtering

In order to find the declared values, implementations must first identify all declarations that apply to each element. A declaration applies to an element if:

The values of the declarations that apply form, for each property on each element, a list of declared values. The next section, the cascade, prioritizes these lists.

6. Cascading

The cascade takes an unordered list of declared values for a given property on a given element, sorts them by their declaration’s precedence as determined below, and outputs a single cascaded value.

6.1. Cascade Sorting Order

The cascade sorts declarations according to the following criteria, in descending order of priority:

Origin and Importance
The origin of a declaration is based on where it comes from and its importance is whether or not it is declared with !important (see below). The precedence of the various origins is, in descending order:
  1. Transition declarations [css-transitions-1]
  2. Important user agent declarations
  3. Important user declarations
  4. Important author declarations
  5. Animation declarations [css-animations-1]
  6. Normal author declarations
  7. Normal user declarations
  8. Normal user agent declarations

Declarations from origins earlier in this list win over declarations from later origins.

A document language can provide for blending declarations sourced from different encapsulation contexts, such as the nested tree contexts of shadow trees in the [DOM].

When comparing two declarations that are sourced from different encapsulation contexts, then for normal rules the declaration from the outer context wins, and for important rules the declaration from the inner context wins. For this purpose, [DOM] tree contexts are considered to be nested in shadow-including tree order.

Note: This effectively means that normal declarations belonging to an encapsulation context can set defaults that are easily overridden by the outer context, while important declarations belonging to an encapsulation context can enforce requirements that cannot be overridden by the outer context.

The Style Attribute
Separately for normal and important declarations, declarations that are attached directly to an element (such as the contents of a style attribute) rather than indirectly mapped by means of a style rule selector take precedence over declarations the same importance that are mapped via style rule.
Declarations within each origin and context can be explicitly assigned to a cascade layer. For the purpose of this step, any declaration not assigned to an explicit layer is added to an implicit final layer.

Cascade layers (like declarations) are ordered by order of appearance. When comparing declarations that belong to different layers, then for normal rules the declaration whose cascade layer is last wins, and for important rules the declaration whose cascade layer is first wins.

Note: This follows the same logic used for layering normal and important origins, so that the !important flag maintains the same “override” purpose in both settings.

The Selectors module [SELECT] describes how to compute the specificity of a selector. Each declaration has the same specificity as the style rule it appears in. The declaration with the highest specificity wins.
Order of Appearance
The last declaration in document order wins. For this purpose:
  • Declarations from imported style sheets are ordered as if their style sheets were substituted in place of the @import rule.
  • Declarations from style sheets independently linked by the originating document are treated as if they were concatenated in linking order, as determined by the host document language.
  • Declarations from style attributes are ordered according to the document order of the element the style attribute appears on, and are all placed after any style sheets.

The output of the cascade is a (potentially empty) sorted list of declared values for each property on each element.

6.2. Cascading Origins

Each style rule has a cascade origin, which determines where it enters the cascade. CSS defines three core origins:

Author Origin
The author specifies style sheets for a source document according to the conventions of the document language. For instance, in HTML, style sheets may be included in the document or linked externally.
User Origin
The user may be able to specify style information for a particular document. For example, the user may specify a file that contains a style sheet or the user agent may provide an interface that generates a user style sheet (or behaves as if it did).
User-Agent Origin
Conforming user agents must apply a default style sheet (or behave as if they did). A user agent’s default style sheet should present the elements of the document language in ways that satisfy general presentation expectations for the document language (e.g., for visual browsers, the EM element in HTML is presented using an italic font). See e.g. the HTML user agent style sheet. [HTML]

Extensions to CSS define the following additional origins:

Animation Origin
CSS Animations [css-animations-1] generate “virtual” rules representing their effects when running.
Transition Origin
Like CSS Animations, CSS Transitions [css-transitions-1] generate “virtual” rules representing their effects when running.

6.3. Important Declarations: the !important annotation

CSS attempts to create a balance of power between author and user style sheets. By default, rules in an author’s style sheet override those in a user’s style sheet, which override those in the user-agent’s default style sheet. To balance this, a declaration can be marked important, which increases its weight in the cascade and inverts the order of precedence.

A declaration is important if it has a !important annotation as defined by [css-syntax-3], i.e. if the last two (non-whitespace, non-comment) tokens in its value are the delimiter token ! followed by the identifier token important. All other declarations are normal (non-important).

[hidden] { display: none !important; }

An important declaration takes precedence over a normal declaration. Author and user style sheets may contain important declarations, with user-origin important declarations overriding author-origin important declarations. This CSS feature improves accessibility of documents by giving users with special requirements (large fonts, color combinations, etc.) control over presentation.

Important declarations from all origins take precedence over animations. This allows authors to override animated values in important cases. (Animated values normally override all other rules.) [css-animations-1]

User-agent style sheets may also contain important declarations. These override all author and user declarations.

The first rule in the user’s style sheet in the following example contains an !important declaration, which overrides the corresponding declaration in the author’s style sheet. The declaration in the second rule will also win due to being marked !important. However, the third declaration in the user’s style sheet is not !important and will therefore lose to the second rule in the author’s style sheet (which happens to set style on a shorthand property). Also, the third author rule will lose to the second author rule since the second declaration is !important. This shows that !important declarations have a function also within author style sheets.
/* From the user’s style sheet */
p { text-indent: 1em !important }
p { font-style: italic !important }
p { font-size: 18pt }

/* From the author’s style sheet */
p { text-indent: 1.5em !important }
p { font: normal 12pt sans-serif !important }
p { font-size: 24pt }
Property Winning value
text-indent 1em
font-style italic
font-size 12pt
font-family sans-serif

6.4. Cascade Layers

In the same way that cascade origins provide a balance of power between user and author styles, cascade layers provide a structured way to organize and balance concerns within a single origin. Authors can create layers to represent element defaults, third-party libraries, themes, components, overrides, and other styling concerns—and are able to re-order the cascade of layers in an explicit way, without altering selectors or specificity within each layer, or relying on source-order to resolve conflicts across layers.

For example, the following generates an explicit defaults layer, with lower cascade weight than any un-layered styles:
audio {
  /* specificity of 0,0,1 - implicit (final) layer */
  display: flex;

@layer defaults {
  audio[controls] {
    /* specificity of 0,1,1 - explicit "defaults" layer */
    display: block;

The un-layered declarations on the audio element take precedence over the explicitly layered declarations on audio[controls]even though the un-layered styles have a lower specificity, and come first in the source order.

6.4.1. Declaring Cascade Layers: the @layer rule

The @layer rule defines an explicit cascade layer, with the option to assign style rules.

The block layer-assignment syntax is:

@layer <layer-ident>? {

Such @layer block rules have the same restrictions and processing as a conditional group rule [CSS-CONDITIONAL-3] with a true condition.

The optional <layer-ident> is a CSS identifier (<ident>) that represents its layer name. If the layer name matches that of a cascade layer already defined within this origin and context (and same layer scope, see § Nested Layers), then its style rules are assigned to that same cascade layer. Otherwise, or if no <layer-ident> is provided, a new cascade layer is created.

Note: Layer name matching does not cross the shadow DOM boundary, so the ordering of layers in the light DOM has no impact on the order of identically-named layers in the shadow DOM.

The @layer rule can also be used to define new layers without assigning any style rules, by providing only a layer name:

@layer <layer-ident>#;

Such empty @layer are allowed anywhere either @import or other @layer rules are allowed.

Provide an attribute for assigning link or style elements to cascade layers <https://github.com/w3c/csswg-drafts/issues/5853>

Layer identifiers are optional, but provide a way to apply multiple style blocks to a single layer. In the following example, the contents of headings.css and links.css are cascaded within the same layer as the audio[controls] rule:
@layer default url(headings.css);
@layer default url(links.css);

@layer default {
  audio[controls] {
    display: block;
The identifier-only syntax allows establishing a layer order in advance, regardless of the order in which style rules are added to each layer.

In this example, the imported theme.css style rules will override any rules added in the later default block since the order of layers has already been established.

@layer default;
@layer theme;
@layer components;

@layer theme url(theme.css);

@layer default {
  audio[controls] {
    display: block;

It can also be written with the comma-separated syntax as:

@layer default, theme, components;

@layer theme url(theme.css);

@layer default {
  audio[controls] {
    display: block;
} Nested Layers

When @layer rules are nested, layer names are scoped to their parent layer.

In this example, the nested framework default layer is distinct from the top-level default layer:
@layer default {
  p { max-width: 70ch; }

@layer framework {
  @layer default {
    p { margin-block: 0.75em; }

  @layer theme {
    p { color: #222; }

The resulting layers can be represented as a tree:

  1. default

  2. framework

    1. default

    2. theme

or as a flat list with nested identifiers:

  1. default

  2. framework default

  3. framework theme

It is not possible for nested layers to reference a layer name in an outer layer’s scope, but it is possible to reference nested layers from an outer scope, by combining identifiers with a full stop (. U+002E) character.

@layer framework {
  @layer default {
     p { margin-block: 0.75em; }

  @layer theme {
    p { color: #222; }

@layer framework.theme {
  /* These styles will be added to the theme layer inside the framework layer */
  blockquote { color: rebeccapurple; }

This syntax is provided as a shorthand for defining nested layers, and has the same effect as declaring each layer name inside nested @layer rules.

That means the shorthand syntax can also be used in defining new layers, and establishing layer order.

The following example defines framework.theme before framework.default:

@layer framework.theme {
  blockquote { color: rebeccapurple; }

@layer framework {
  @layer default {
     p { margin-block: 0.75em; }

  @layer theme {
    p { color: #222; }

The framework.theme shorthand is purely syntax sugar for the following longhand:

@layer framework {
  @layer theme {
    blockquote { color: rebeccapurple; }

What is the appropriate syntax for appending to nested layers? <https://github.com/w3c/csswg-drafts/issues/5791> Un-Named Layers
A layer declared without a layer name does not provide any external hook for re-arranging or adding styles.

In most use-cases this would only be syntax-sugar for brevity —relying on well-organized source-order rather than any explicit names. However, it could be used by teams as a way to "force" an organizing convention (all layer code must be defined in one place), or by libraries wanting to merge & hide a set of internal "private" layers that they don’t want exposed to author manipulation:

/* bootstrap-base.css */
/* unnamed wrapper layers around each sub-file */
@layer url(base-forms.css);
@layer url(base-links.css);
@layer url(base-headings.css);

/* bootstrap.css */
/* the internal names are hidden from access, subsumed in "base" */
@layer base url(bootstrap-base.css);

/* author.css */
/* author has access to bootstrap.base layer, but not into unnamed layers */
@layer bootstrap url(bootstrap.css);

6.5. Precedence of Non-CSS Presentational Hints

The UA may choose to honor presentational hints in a source document’s markup, for example the bgcolor attribute or s element in [HTML]. All document language-based styling must be translated to corresponding CSS rules and either enter the cascade as UA-origin rules or be treated as author-origin rules in their own cascade layer placed as the lowest layer in the author origin. A document language may define whether such a presentational hint enters the cascade as UA-origin or author-origin; if so, the UA must behave accordingly. For example, [SVG11] maps its presentation attributes into the author origin.

Note: Presentational hints entering the cascade as UA-origin rules can be overridden by author-origin or user-origin styles. Presentational hints entering the cascade as author-origin rules can be overridden by author-origin styles, but not by non-important user-origin styles. Host languages should choose the appropriate origin for presentational hints with these considerations in mind.

7. Defaulting

When the cascade does not result in a value, the specified value must be found some other way. Inherited properties draw their defaults from their parent element through inheritance; all other properties take their initial value. Authors can explicitly request inheritance or initialization via the inherit and initial keywords.

7.1. Initial Values

Each property has an initial value, defined in the property’s definition table. If the property is not an inherited property, and the cascade does not result in a value, then the specified value of the property is its initial value.

7.2. Inheritance

Inheritance propagates property values from parent elements to their children. The inherited value of a property on an element is the computed value of the property on the element’s parent element. For the root element, which has no parent element, the inherited value is the initial value of the property.

For a [DOM] tree with shadows, inheritance operates on the flattened element tree. This means that slotted elements inherit from the slot they’re assigned to, rather than directly from their light tree parent. Pseudo-elements inherit according to the fictional tag sequence described for each pseudo-element. [CSS-PSEUDO-4]

Some properties are inherited properties, as defined in their property definition table. This means that, unless the cascade results in a value, the value will be determined by inheritance.

A property can also be explicitly inherited. See the inherit keyword.

Note: Inheritance follows the document tree and is not intercepted by anonymous boxes, or otherwise affected by manipulations of the box tree.

7.3. Explicit Defaulting

Several CSS-wide property values are defined below; declaring a property to have these values explicitly specifies a particular defaulting behavior. As specified in CSS Values and Units [css-values-3], all CSS properties can accept these values.

7.3.1. Resetting a Property: the initial keyword

If the cascaded value of a property is the initial keyword, the property’s specified value is its initial value.

7.3.2. Explicit Inheritance: the inherit keyword

If the cascaded value of a property is the inherit keyword, the property’s specified and computed values are the inherited value.

7.3.3. Erasing All Declarations: the unset keyword

If the cascaded value of a property is the unset keyword, then if it is an inherited property, this is treated as inherit, and if it is not, this is treated as initial. This keyword effectively erases all declared values occurring earlier in the cascade, correctly inheriting or not as appropriate for the property (or all longhands of a shorthand).

7.3.4. Rolling Back Cascade Origins: the revert keyword

If the cascaded value of a property is the revert keyword, the behavior depends on the cascade origin to which the declaration belongs:

user-agent origin
Equivalent to unset.
user origin
Rolls back the cascaded value to the user-agent level, so that the specified value is calculated as if no author-origin or user-origin rules were specified for this property on this element.
author origin
Rolls back the cascaded value to the user level, so that the specified value is calculated as if no author-origin rules were specified for this property on this element. For the purpose of revert, this origin includes the Animation origin.

7.3.5. Rolling Back Cascade Layers: the revert-layer keyword

If the cascaded value of a property is the revert-layer keyword, the cascaded value is rolled back to the layer below, so that the specified value is calculated as if no rules were specified in the current cascade layeror between its normal and important levels in the cascadefor this property on this element.

Note: If there are no lower-priority declarations in the same cascade origin as the revert-layer value, the cascaded value will roll back to the previous origin, just as for the revert keyword.

8. Changes

Changes since the 19 January 2021 First Public Working Draft include:

8.1. Additions Since Level 4

The following features have been added since Level 4:

8.2. Additions Since Level 3

The following features have been added since Level 3:

8.3. Additions Since Level 2

The following features have been added since Level 2:


David Baron, Tantek Çelik, Florian Rivoal, Simon Sapin, Jen Simmons, and Boris Zbarsky contributed to this specification.

Privacy and Security Considerations


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.
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.

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 component 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.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

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 https://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.


Terms defined by this specification

Terms defined by reference


Normative References

Dean Jackson; et al. CSS Animations Level 1. 11 October 2018. WD. URL: https://www.w3.org/TR/css-animations-1/
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 22 December 2020. CR. URL: https://www.w3.org/TR/css-backgrounds-3/
Tantek Çelik; Chris Lilley; David Baron. CSS Color Module Level 3. 19 June 2018. REC. URL: https://www.w3.org/TR/css-color-3/
Tab Atkins Jr.; Chris Lilley. CSS Color Module Level 4. 12 November 2020. WD. URL: https://www.w3.org/TR/css-color-4/
David Baron; Elika Etemad; Chris Lilley. CSS Conditional Rules Module Level 3. 8 December 2020. CR. URL: https://www.w3.org/TR/css-conditional-3/
Tab Atkins Jr.; Elika Etemad. CSS Display Module Level 3. 18 December 2020. CR. URL: https://www.w3.org/TR/css-display-3/
John Daggett; Myles Maxfield; Chris Lilley. CSS Fonts Module Level 3. 20 September 2018. REC. URL: https://www.w3.org/TR/css-fonts-3/
Daniel Glazman; Elika Etemad; Alan Stearns. CSS Pseudo-Elements Module Level 4. 31 December 2020. WD. URL: https://www.w3.org/TR/css-pseudo-4/
Tab Atkins Jr.; Elika Etemad. CSS Scoping Module Level 1. 3 April 2014. WD. URL: https://www.w3.org/TR/css-scoping-1/
Tab Atkins Jr.; Simon Sapin. CSS Syntax Module Level 3. 16 July 2019. CR. URL: https://www.w3.org/TR/css-syntax-3/
David Baron; et al. CSS Transitions. 11 October 2018. WD. URL: https://www.w3.org/TR/css-transitions-1/
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 6 June 2019. CR. URL: https://www.w3.org/TR/css-values-3/
Tab Atkins Jr.. CSS Custom Properties for Cascading Variables Module Level 1. 3 December 2015. CR. URL: https://www.w3.org/TR/css-variables-1/
Elika Etemad; Koji Ishii. CSS Writing Modes Level 3. 10 December 2019. REC. URL: https://www.w3.org/TR/css-writing-modes-3/
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: https://www.w3.org/TR/CSS21/
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
Anne van Kesteren. Fetch Standard. Living Standard. URL: https://fetch.spec.whatwg.org/
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
Florian Rivoal; Tab Atkins Jr.. Media Queries Level 4. 21 July 2020. CR. URL: https://www.w3.org/TR/mediaqueries-4/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
Tantek Çelik; et al. Selectors Level 3. 6 November 2018. REC. URL: https://www.w3.org/TR/selectors-3/
Elika Etemad; Tab Atkins Jr.. Selectors Level 4. 21 November 2018. WD. URL: https://www.w3.org/TR/selectors-4/

Informative References

Elika Etemad; Tab Atkins Jr.. CSS Box Alignment Module Level 3. 21 April 2020. WD. URL: https://www.w3.org/TR/css-align-3/
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 3. 4 December 2018. CR. URL: https://www.w3.org/TR/css-break-3/
Rossen Atanassov; Elika Etemad. CSS Fragmentation Module Level 4. 18 December 2018. WD. URL: https://www.w3.org/TR/css-break-4/
Tab Atkins Jr.; et al. CSS Flexible Box Layout Module Level 1. 19 November 2018. CR. URL: https://www.w3.org/TR/css-flexbox-1/
Dave Cramer; Elika Etemad; Steve Zilles. CSS Inline Layout Module Level 3. 27 August 2020. WD. URL: https://www.w3.org/TR/css-inline-3/
Elika Etemad; Tab Atkins Jr.. CSS Lists and Counters Module Level 3. 17 November 2020. WD. URL: https://www.w3.org/TR/css-lists-3/
Elika Etemad; Simon Sapin. CSS Paged Media Module Level 3. 18 October 2018. WD. URL: https://www.w3.org/TR/css-page-3/
Tab Atkins Jr.; Elika Etemad. CSS Box Sizing Module Level 3. 18 December 2020. WD. URL: https://www.w3.org/TR/css-sizing-3/
Elika Etemad; Koji Ishii; Florian Rivoal. CSS Text Module Level 3. 22 December 2020. CR. URL: https://www.w3.org/TR/css-text-3/
Tantek Çelik; Florian Rivoal. CSS Basic User Interface Module Level 3 (CSS3 UI). 21 June 2018. REC. URL: https://www.w3.org/TR/css-ui-3/
Elika Etemad; Koji Ishii. CSS Writing Modes Level 4. 30 July 2019. CR. URL: https://www.w3.org/TR/css-writing-modes-4/
Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-1/
Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. REC. URL: https://www.w3.org/TR/SVG11/

Property Index

Name Value Initial Applies to Inh. %ages Anim­ation type Canonical order Com­puted value
all initial | inherit | unset | revert | revert-layer see individual properties see individual properties see individual properties see individual properties see individual properties per grammar see individual properties

Issues Index

Provide an attribute for assigning link or style elements to cascade layers <https://github.com/w3c/csswg-drafts/issues/5853>
What is the appropriate syntax for appending to nested layers? <https://github.com/w3c/csswg-drafts/issues/5791>