CSS Conditional Rules Module Level 3

1. Introduction

1.1. Background

This section is not normative.

[CSS21] defines one type of conditional group rule, the @media rule, and allows only style rules (not other @-rules) inside of it. The @media rule provides the ability to have media-specific style sheets, which is also provided by style sheet linking features such as @import and link. The restrictions on the contents of @media rules made them less useful; they have forced authors using CSS features involving @-rules in media-specific style sheets to use separate style sheets for each medium.

This specification extends the rules for the contents of conditional group rules to allow other @-rules, which enables authors to combine CSS features involving @-rules with media specific style sheets within a single style sheet.

This specification also defines an additional type of conditional group rule, @supports, to address author and user requirements.

The @supports rule allows CSS to be conditioned on implementation support for CSS properties and values. This rule makes it much easier for authors to use new CSS features and provide good fallback for implementations that do not support those features. This is particularly important for CSS features that provide new layout mechanisms, and for other cases where a set of related styles needs to be conditioned on property support.

1.2. Module Interactions

This module replaces and extends the @media rule feature defined in [CSS21] section 7.2.1 and incorporates the modifications previously made non-normatively by [MEDIAQUERIES-4] section 1.

2. Processing of conditional group rules

This specification defines some CSS at-rules, called conditional group rules, that associate a condition with a group of other CSS rules. These different rules allow testing different types of conditions, but share common behavior for how their contents are used when the condition is true and when the condition is false.

@media print {
  /* hide navigation controls when printing */
  #navigation { display: none }
}

Each conditional group rule has a condition, which at any time evaluates to true or false. When the condition is true, CSS processors must apply the rules inside the group rule as though they were at the group rule’s location; when the condition is false, CSS processors must not apply any of rules inside the group rule. The current state of the condition does not affect the CSS object model, in which the contents of the group rule always remain within the group rule.

This means that when multiple conditional group rules are nested, a rule inside of both of them applies only when all of the rules' conditions are true.

@media print { /* rule (1) */
  /* hide navigation controls when printing */
  #navigation { display: none }
  @media (max-width: 12cm) { /* rule (2) */
    /* keep notes in flow when printing to narrow pages */
    .note { float: none }
  }
}

When the condition for a conditional group rule changes, CSS processors must reflect that the rules now apply or no longer apply, except for properties whose definitions define effects of computed values that persist past the lifetime of that value (such as for some properties in [CSS3-TRANSITIONS] and [CSS3-ANIMATIONS]).

3. Contents of conditional group rules

All conditional group rules are defined to take a <stylesheet> in their block, which means they can accept any rule that is normally allowed at the top-level of a stylesheet, and not otherwise restricted. (For example, an @import rule must appear at the actual beginning of a stylesheet, and so is not valid inside of another rule.)

Invalid or unknown rules inside the <stylesheet> must be considered invalid and ignored, but do not invalidate the conditional group rule.

Any namespace prefixes used in a conditional group rule must have been declared, otherwise they are invalid.

@namespace x url(http://www.w3.org/1999/xlink);
@supports (content: attr(x|href)) {
  // do something }

@supports (content: attr(n|tooltip)) {
  // do something }

4. Placement of conditional group rules

Conditional group rules are allowed wherever style rules are allowed (at the top-level of a style sheet, as well as within other conditional group rules). CSS processors must process such rules as described above.

Any at-rules that are not allowed after a style rule (e.g., @charset, @import, or @namespace rules) are also not allowed after a conditional group rule, and are therefore invalid when so placed.

5. Media-specific style sheets: the @media rule

The @media rule is a conditional group rule whose condition is a media query. Its syntax is:

@media <media-query-list> {
  <stylesheet>
}

It consists of the at-keyword @media followed by a (possibly empty) media query list (as defined in [MEDIAQUERIES-4]), followed by a block containing arbitrary rules. The condition of the rule is the result of the media query.

@media screen and (min-width: 35em),
       print and (min-width: 40em) {
  #section_navigation { float: left; width: 10em; }
}

6. Feature queries: the @supports rule

The @supports rule is a conditional group rule whose condition tests whether the user agent supports CSS property:value pairs. Authors can use it to write style sheets that use new features when available but degrade gracefully when those features are not supported. These queries are called CSS feature queries or (colloquially) supports queries.

Note: CSS has existing mechanisms for graceful degradation, such as ignoring unsupported properties or values, but these are not always sufficient when large groups of styles need to be tied to the support for certain features, as is the case for use of new layout system features.

The syntax of the condition in the @supports rule is similar to that defined for <media-condition> in [MEDIAQUERIES-4], but without the "unknown" value logic:

• negation is needed so that the new-feature styles and the fallback styles can be separated (within the forward-compatible grammar’s rules for the syntax of @-rules), and not required to override each other.

• conjunction (and) is needed so that multiple required features can be tested.

• disjunction (or) is needed when there are multiple alternative features for a set of styles, particularly when some of those alternatives are vendor-prefixed properties or values.

Therefore, the syntax of the @supports rule allows testing for property:value pairs, and arbitrary conjunctions (and), disjunctions (or), and negations (not) of them.

The syntax of the @supports rule is:

@supports <supports-condition> {
  <stylesheet>
}

with <supports-condition> defined as:

<supports-condition> = not <supports-in-parens>
                     | <supports-in-parens> [ and <supports-in-parens> ]*
                     | <supports-in-parens> [ or <supports-in-parens> ]*
<supports-in-parens> = ( <supports-condition> ) | <supports-feature> | <general-enclosed>
<supports-feature> = <supports-decl>
<supports-decl> = ( <declaration> )

The above grammar is purposely very loose for forwards-compatibility reasons, since the <general-enclosed> production allows for substantial future extensibility. Any @supports rule that does not parse according to the grammar above (that is, a rule that does not match this loose grammar which includes the <general-enclosed> production) is invalid. Style sheets must not use such a rule and processors must ignore such a rule (including all of its contents).

Each of these grammar terms is associated with a boolean result, as follows:

<supports-condition>

<supports-in-parens>

The result is the result of the child subexpression.

not <supports-in-parens>

The result is the negation of the <supports-in-parens> term.

<supports-in-parens> [ and <supports-in-parens> ]*

The result is true if all of the <supports-in-parens> child terms are true, and false otherwise.

<supports-in-parens> [ or <supports-in-parens> ]*

The result is false if all of the <supports-in-parens> child terms are false, and true otherwise.

<supports-decl>

The result is true if the UA supports the declaration within the parentheses.

<general-enclosed>

The result is false.

Authors must not use <general-enclosed> in their stylesheets. It exists only for future-compatibility, so that new syntax additions do not invalidate too much of a <supports-condition> in older user agents.

The condition of the @supports rule is the result of the <supports-condition> in its prelude.

@supports ( display: flex ) {
  body, #navigation, #content { display: flex; }
  #navigation { background: blue; color: white; }
  #article { background: white; color: black; }
}

@supports not ( display: flex ) {
  body { width: 100%; height: 100%; background: white; color: black; }
  #navigation { width: 25%; }
  #article { width: 75%; }
}

.noticebox {
  border: 1px solid black;
  padding: 1px;
}
@supports ( box-shadow: 0 0 2px black inset ) or
          ( -moz-box-shadow: 0 0 2px black inset ) or
          ( -webkit-box-shadow: 0 0 2px black inset ) or
          ( -o-box-shadow: 0 0 2px black inset ) {
  .noticebox {
    -moz-box-shadow: 0 0 2px black inset;
    -webkit-box-shadow: 0 0 2px black inset;
    -o-box-shadow: 0 0 2px black inset;
    box-shadow: 0 0 2px black inset; /* unprefixed last */
    /* override the rule above the @supports rule */
    border: none;
    padding: 2px;
  }
}

To avoid confusion between and and or, the syntax requires that both and and or be specified explicitly (rather than, say, using commas or spaces for one of them). Likewise, to avoid confusion caused by precedence rules, the syntax does not allow and, or, and not operators to be mixed without a layer of parentheses.

@supports (transition-property: color) or
          (animation-name: foo) and
          (transform: rotate(10deg)) {
  /* ... */
}

@supports ((transition-property: color) or
           (animation-name: foo)) and
          (transform: rotate(10deg)) {
  /* ... */
}

@supports (transition-property: color) or
          ((animation-name: foo) and
           (transform: rotate(10deg))) {
  /* ... */
}

The declaration being tested must always occur within parentheses, when it is the only thing in the expression.

@supports display: flex {
  /* ... */
}

@supports (display: flex) {
  /* ... */
}

The syntax allows extra parentheses when they are not needed. This flexibility is sometimes useful for authors (for example, when commenting out parts of an expression) and may also be useful for authoring tools.

@supports ((display: flex)) {
  /* ... */
}

A trailing !important on a declaration being tested is allowed, though it won’t change the validity of the declaration.

@supports (display: flex !important) {
  /* ... */
}

6.1. Definition of support

For forward-compatibility, section 4.1.8 (Declarations and properties) of [CSS21] defines rules for handling invalid properties and values. CSS processors that do not implement or partially implement a specification must treat any part of a value that they do not implement, or do not have a usable level of support for, as invalid according to this rule for handling invalid properties and values, and therefore must discard the declaration as a parse error.

A CSS processor is considered to support a declaration (consisting of a property and value) if it accepts that declaration (rather than discarding it as a parse error) within a style rule. If a processor does not implement, with a usable level of support, the value given, then it must not accept the declaration or claim support for it.

Note: Note that properties or values whose support is effectively disabled by user preferences are still considered as supported by this definition. For example, if a user has enabled a high-contrast mode that causes colors to be overridden, the CSS processor is still considered to support the color property even though declarations of the color property may have no effect. On the other hand, a developer-facing preference whose purpose is to enable or disable support for an experimental CSS feature does affect this definition of support.

These rules (and the equivalence between them) allow authors to use fallback (either in the [CSS1] sense of declarations that are overridden by later declarations or with the new capabilities provided by the @supports rule in this specification) that works correctly for the features implemented. This applies especially to compound values; implementations must implement all parts of the value in order to consider the declaration supported, either inside a style rule or in the declaration condition of an @supports rule.

7. APIs

7.1. Extensions to the CSSRule interface

The CSSRule interface is extended as follows:

partial interface CSSRule {
    const unsigned short SUPPORTS_RULE = 12;
};

7.2. The CSSConditionRule interface

The CSSConditionRule interface represents all the “conditional” at-rules, which consist of a condition and a statement block.

[Exposed=Window]
interface CSSConditionRule : CSSGroupingRule {
    attribute CSSOMString conditionText;
};

conditionText of type CSSOMString

The conditionText attribute represents the condition of the rule. Since what this condition does varies between the derived interfaces of CSSConditionRule, those derived interfaces may specify different behavior for this attribute (see, for example, CSSMediaRule below). In the absence of such rule-specific behavior, the following rules apply:

The conditionText attribute, on getting, must return the result of serializing the associated condition.

On setting the conditionText attribute these steps must be run:

1. Trim the given value of white space.
2. If the given value parses as the appropriate condition grammar for the given rule (such as <supports-condition> for @supports, etc), replace the associated CSS condition with the given value.
3. Otherwise, do nothing.

7.3. The CSSMediaRule interface

The CSSMediaRule interface represents a @media at-rule:

[Exposed=Window]
interface CSSMediaRule : CSSConditionRule {
    [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
};

media of type MediaList, readonly

The media attribute must return a MediaList object for the list of media queries specified with the @media at-rule.

conditionText of type CSSOMString (CSSMediaRule-specific definition for attribute on CSSConditionRule)

The conditionText attribute (defined on the CSSConditionRule parent rule), on getting, must return the value of media.mediaText on the rule.

Setting the conditionText attribute must set the media.mediaText attribute on the rule.

7.4. The CSSSupportsRule interface

The CSSSupportsRule interface represents a @supports rule.

[Exposed=Window]
interface CSSSupportsRule : CSSConditionRule {
};

conditionText of type CSSOMString (CSSSupportsRule-specific definition for attribute on CSSConditionRule)

The conditionText attribute (defined on the CSSConditionRule parent rule), on getting, must return the condition that was specified, without any logical simplifications, so that the returned condition will evaluate to the same result as the specified condition in any conformant implementation of this specification (including implementations that implement future extensions allowed by the <general-enclosed> extensibility mechanism in this specification). In other words, token stream simplifications are allowed (such as reducing whitespace to a single space or omitting it in cases where it is known to be optional), but logical simplifications (such as removal of unneeded parentheses, or simplification based on evaluating results) are not allowed.

7.5. The CSS namespace, and the supports() function

The CSS namespace holds useful CSS-related functions that do not belong elsewhere.

partial namespace CSS {
  boolean supports(CSSOMString property, CSSOMString value);
  boolean supports(CSSOMString conditionText);
};

supports(CSSOMString property, CSSOMString value), returns boolean

When the supports(property, value) method is invoked with two arguments property and value:

1. If property is an ASCII case-insensitive match for any defined CSS property that the UA supports, and value successfully parses according to that property’s grammar, return true.

2. Otherwise, if property is a custom property name string, return true.

3. Otherwise, return false.

Note: No CSS escape or whitespace processing is performed on the property name, so CSS.supports(" width", "5px") will return false, as " width" isn’t the name of any property due to the leading space.

Note: !important flags are not part of property grammars, and will cause value to parse as invalid, just as they would in the value argument to element.style.setProperty().

supports(CSSOMString conditionText), returns boolean

When the supports(conditionText) method is invoked with a single conditionText argument:

1. If conditionText, parsed and evaluated as a <supports-condition>, would return true, return true.

2. Otherwise, If conditionText, wrapped in parentheses and then parsed and evaluated as a <supports-condition>, would return true, return true.

3. Otherwise, return false.

All namespaces in the conditionText argument are considered invalid, just as they are in document.querySelector("a|b").

Security Considerations

This spec introduces no new security considerations.

Privacy Considerations

Various features in this specification, associated mainly with the @media rule but also to some degree with the @supports rule, provide information to Web content about the user’s hardware and software and their configuration and state. Most of the information is provided through the features in [MEDIAQUERIES-4] rather than through the features in this specification. However, the @supports rule may provide some additional details about the user’s software and whether it is running with non-default settings that may enable or disable certain features.

Most of this information can also be determined through other APIs. However, the features in this specification are one of the ways this information is exposed on the Web.

This information can also, in aggregate, be used to improve the accuracy of fingerprinting of the user.

8. Changes

The following (non-editorial) changes were made to this specification since the 8 December 2020 Candidate Recommendation Snapshot:

• Defined the terms CSS feature queries and supports queries to refer to the conditional syntax of the @supports rules, to allow better cross-referencing.
• Removed the “unknown” value in CSS feature queries’ boolean logic, defining unrecognized syntaxes as “false” instead. (Issue 6175)
• Clarified placement of conditional group rules. (Issue 5697)

  Conditional group rules are allowed wherever style rules are allowed ( at the top-level of a style sheet, and inside as well as within other conditional group rules ) . CSS processors must process such rules as described above.

  Any at- rules that are not allowed after a style rule (e.g., @charset , @import , or @namespace rules) are also not allowed after a conditional group rule . Therefore, style sheets must not place such rules after a conditional group rule, and CSS processors must ignore such rules. , and are therefore invalid when so placed.

The following (non-editorial) changes were made to this specification since the 4 April 2013 Candidate Recommendation:

• Clarified that namespaces in conditionText are invalid
• New editors added
• Added explicit call to parse rather than "matches the grammar"
• Removed duplicate CSSGroupingRule, which is already defined by CSSOM
• Rewrote the supports() text into algorithm form, to make it easier to express that you pay attention to the syntax of registered custom properties in the supports(prop, val) form.
• Moved the definition of @supports selector to css-conditional-4.
• @supports' is no longer at risk.
• Rewrote to use CSS Syntax grammars, not CSS 2.1 grammars
• Changed from CSS Interface to WebIDL-compatible CSS namespace
• Dropped requirement for spaces around and, or, and not keywords for consistency with Media Queries (which are themselves constrained by compatibility with the output of some CSS minimizers that rely on some of the more arcane aspects of CSS tokenization). Note that white space--or a comment--is still required after these keywords, since without it they and the ensuing opening parenthesis will be tokenized as a function opening token.
• Allowed the supports() method to imply parentheses for simple declarations, for consistency with the @import rule’s supports() function.
• Fixed missing semicolons in IDL code.
• Updated links, terminology, and example code in response to changes to other modules.
• Spelling and grammatical corrections
• Added section on privacy and security considerations.

Acknowledgments

Thanks to the ideas and feedback from Tab Atkins, Arthur Barstow, Ben Callahan, Tantek Çelik, Alex Danilo, Elika Etemad, Pascal Germroth, Björn Höhrmann, Paul Irish, Brad Kemper, Anne van Kesteren, Vitor Menezes, Alex Mogilevsky, Chris Moschini, James Nurthen, Simon Pieters, Florian Rivoal, Simon Sapin, Nicholas Shanks, Ben Ward, Zack Weinberg, Estelle Weyl, Boris Zbarsky, and all the rest of the www-style community.