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[css-conditional-4][css-conditional-5] Split Level 4 ED
* Existing feature (selector()) from FPWD, which is implemented and shipping, to go to CR in L4 * New features (font-*(), @when, @else) which were never published on /TR to go to FPWD in L5
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‎css-conditional-4/Overview.bs

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@@ -13,336 +13,95 @@ Test Suite: http://test.csswg.org/suites/css-conditional-4_dev/nightly-unstable/
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Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
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Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
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Editor: Chris Lilley, W3C, https://svgees.us/, w3cid 1438
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Abstract: This module contains the features of CSS for conditional processing of parts of
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style sheets, conditioned on capabilities of the processor or the
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document the style sheet is being applied to. It includes and extends the
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functionality of CSS lConditional 3 [[!css-conditional-3]].
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The main extensions compared to level 3 are
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the Generalized Conditional ''@when''
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and Chained Conditional ''@else'' rules, and
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the addition of selector and font tech queries to the the ''@supports'' rule.
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Abstract: This module contains the features of CSS
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for conditional processing of parts of style sheets,
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based on capabilities of the processor or the environment
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the style sheet is being applied in.
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It includes and extends the functionality of CSS Conditional 3 [[!css-conditional-3]],
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adding the ability to query support for particular selectors [[SELECTORS-4]]
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through the new ''selector()'' notation for [=supports queries=].
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Default Highlight: css
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</pre>
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<pre class="link-defaults">
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spec: css-conditional-3
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type: dfn; text: conditional group rule
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spec: css-conditional-3
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type: dfn; text: conditional group rule
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</pre>
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<h2 id="introduction">Introduction</h2>
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This is currently an early draft of the things that are <em>new</em> in level 4.
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The features in level 3 are still defined in [[css-conditional-3]]
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and have not yet been copied here.
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ISSUE: The features in level 3 are still defined in [[css-conditional-3]]
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and have not yet been copied here.
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It adds extensions to the ''@supports'' rule
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to allow testing for supported selectors
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and supported font technologies.
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This level adds extensions to the ''@supports'' rule
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to allow testing for supported selectors.
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It also adds an ''@when'' rule,
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which generalizes the concept of a conditional rule.
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Anything you can express in an existing conditional rule
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can be expressed in ''@when'',
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it just has to be wrapped in an appropriate function
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to declare what kind of condition it is.
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This allow authors to easily combine multiple types of queries,
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such as media queries and supports queries,
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in a single boolean expression.
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Without this,
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authors must rely on nesting separate conditional rules,
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which is harder to read and write,
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presupposes the conditions are to be conjoined with the "and" boolean relation
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(with no easy way to indicate anything else),
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and restricts their utility in the proposed <a>conditional rule chains</a>.
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<h2 id="at-supports-ext">
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Extensions to the ''@supports'' rule</h2>
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It also adds ''@else'' rules,
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which immediately follow conditional rules
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and automatically qualify their conditions as you'd expect,
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such that at <em>most one rule</em> in an <a>conditional rule chain</a> is chosen as active.
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This level of the specification extends the <<supports-feature>> syntax as follows:
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Issue: In the future, copy the contents of [[css-conditional-3]] into this document.
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<h2 id="at-supports-ext">Extensions to the ''@supports'' rule</h2>
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This level of the specification extends the <<supports-feature>> syntax as follows:
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<pre class="prod def" nohighlight>
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<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-font-tech-fn>>
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| <<supports-font-format-fn>> | <<supports-decl>>
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<dfn>&lt;supports-selector-fn></dfn> = selector( <<complex-selector>> )
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<dfn>&lt;supports-font-tech-fn></dfn> = font-tech ( <<font-tech>> )
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<dfn>&lt;font-tech></dfn> = [ features-opentype | features-aat | features-graphite
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| color-colrv0 | color-colrv1 | color-svg | color-sbix | color-cbdt
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| variations | palettes | incremental ]
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<dfn>&lt;supports-font-format-fn></dfn> = font-format ( <<font-format>> )
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<dfn>&lt;font-format</dfn> = [ collection | embedded-opentype | opentype
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| svg | truetype | woff | woff2 ]
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</pre>
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: <<supports-selector-fn>>
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::
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The result is true if the UA
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<a href="#dfn-support-selector">supports the selector</a>
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provided as an argument to the function.
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: <<supports-font-tech-fn>>
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::
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The result is true if the UA
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<a href="#dfn-support-font-tech">supports the font tech</a>
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provided as an argument to the function.
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: <<supports-font-format-fn>>
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::
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The result is true if the UA
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<a href="#dfn-support-font-format">supports the font format</a>
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provided as an argument to the function.
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<h3 id="support-definition-ext">Extensions to the definition of support</h3>
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A CSS processor is considered to
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<dfn export for=CSS id="dfn-support-selector">support a CSS selector</dfn>
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if it accepts that selector (rather than discarding it as a
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parse error), and that selector doesn't contain
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<a>unknown -webkit- pseudo-elements</a>.
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A CSS processor is considered to
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<dfn export for=CSS id="dfn-support-font-tech">support a font tech</dfn>
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when the text and layout processing engine ingesting
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this CSS text is capable of utilising the specified [[css-fonts-4#font-tech-definitions]]
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in layout and rendering.
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Note: the allowed values for the font-tech function
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are the same as those for the tech function
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in the '@font-face' '@font-face/src' descriptor.
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A CSS processor is considered to
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<dfn export for=CSS id="dfn-support-font-format">support a font format</dfn>
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when the text and layout processing engine ingesting
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this CSS text is capable of utilising the specified [[css-fonts-4#font-format-definitions]]
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in layout and rendering.
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Note: the allowed values for the font-format function
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are the same as those for the format function
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in the '@font-face' '@font-face/src' descriptor.
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<h2 id="when-rule">Generalized Conditional Rules: the ''@when'' rule</h2>
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The <dfn>@when</dfn> at-rule is a <a>conditional group rule</a>
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that generalizes the individual <a>conditional group rules</a>
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such as ''@media'' and ''@supports''.
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It is defined as:
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<pre class=prod>
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@when <<boolean-condition>> {
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<<stylesheet>>
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}
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</pre>
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Where <<boolean-condition>> is a boolean algebra a la [[mediaqueries-4#mq-syntax]],
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but with ''media()'' and ''supports()'' functions as leaves.
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Issue: Define "boolean algebra, with X as leaves" in a generic way in Conditional,
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so all the conditional rules can reference it directly,
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rather than having to redefine boolean algebra on their own.
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The ''media()'' and ''supports()'' functions are defined as:
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<pre class=prod>
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<dfn>media()</dfn> = media( [ <<mf-plain>> | <<mf-boolean>> | <<mf-range>> ] )
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<dfn>supports()</dfn> = supports( <<declaration>> )
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</pre>
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A ''media()'' or ''supports()'' function is associated the boolean result
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that its contained condition is associated with.
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<h2 id="else-rule">Chained Conditionals: the ''@else'' rule</h2>
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Usually, <a>conditional group rules</a> are independent;
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each one has a separate condition
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evaluated without direct reference to any other rule,
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and decides whether or not to apply its contained rules
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based solely on its condition.
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This is fine for simple conditions,
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but makes it difficult to write a collection of conditionals that are meant to be mutually exclusive;
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authors have to very carefully craft their conditions to not activate when the other rules are meant to,
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and make sure the collection of conditionals don't accidentally <em>all</em> exclude some situation
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which is then left unstyled.
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The <dfn>@else</dfn> rule is a <a>conditional group rule</a>
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used to form <a>conditional rule chains</a>,
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which allow multiple conditional rules to be provided
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and guarantee that at most one of them will evaluate their condition as true.
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It is defined as:
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<pre class=prod>
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@else <<boolean-condition>>? {
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<<stylesheet>>
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}
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</pre>
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''@else'' is interpreted identically to ''@when''.
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If its <<boolean-condition>> is omitted,
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it's treated as having a condition that's always true.
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A <dfn export>conditional rule chain</dfn> is a series of consecutive <a>conditional group rules</a>,
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starting with a <a>conditional group rule</a> other than ''@else'',
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followed by zero or more ''@else'' rules.
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There cannot be anything between the successive <a>conditional group rules</a>
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other than whitespace and/or comments;
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any other token "breaks" the chain.
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Issue: Should we require that only the last ''@else'' in a chain can have an omitted condition?
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It's not uncommon for me, when debugging code,
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to short-circuit an if-else chain by setting one of them to "true";
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I presume that would be similarly useful in CSS?
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It's still pretty easy to see you've done something wrong if you omit the condition accidentally.
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Within a <a>conditional rule chain</a>,
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the conditions of each <a>conditional group rule</a> are evaluated in order.
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If one of them is true,
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the conditions of all <em>following</em> <a>conditional group rules</a> in the chain must evaluate to false,
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regardless of what they contain.
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An ''@else'' rule that is not part of a <a>conditional rule chain</a> is invalid and must be ignored.
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<div class=example>
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For example, here's a (somewhat silly) conditional chain:
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<pre class="lang-css">
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@when media(width >= 400px) and media(pointer: fine) and supports(display: flex) {
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/* A */
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} @else supports(caret-color: pink) and supports(background: double-rainbow()) {
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/* B */
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} @else {
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/* C */
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}
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<pre class="prod def" nohighlight>
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<dfn>&lt;supports-feature></dfn> = <<supports-selector-fn>> | <<supports-decl>>
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<dfn>&lt;supports-selector-fn></dfn> = selector( <<complex-selector>> )
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</pre>
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Exactly one of the preceding rules will be chosen,
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even though the second rule
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doesn't exclude large widths, fine points, or flexbox support,
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and the last rule
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doesn't specify anything at all.
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To achieve the same result without <a>conditional rule chains</a>,
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you'd need to write:
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: <<supports-selector-fn>>
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::
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The result is true if the UA
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<a href="#dfn-support-selector">supports the selector</a>
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provided as an argument to the function.
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<pre class="lang-css">
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@media (width >= 400px) and (pointer: fine) {
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@supports (display: flex) {
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/* A */
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}
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@supports not (display: flex) {
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@supports (caret-color: pink) and (background: double-rainbow()) {
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/* B */
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}
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@supports not ((caret-color: pink) and (background: double-rainbow())) {
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/* C */
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}
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}
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}
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@media not ((width >= 400px) and (pointer: fine)) {
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@supports (caret-color: pink) and (background: double-rainbow()) {
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/* B */
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}
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@supports not ((caret-color: pink) and (background: double-rainbow())) {
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/* C */
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}
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}
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</pre>
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<h3 id="support-definition-ext">
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Extensions to the definition of support</h3>
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This is simultaneously impossible to read,
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requires significant duplication of both conditions and contents,
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and is <em>very</em> difficult to write correctly
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<!-- (I wrote it wrong <em>twice</em> while producing this example). -->
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If the conditions got any more complicated
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(which is not unusual in real-world content),
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the example would get <em>significantly</em> worse.
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</div>
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A CSS processor is considered to
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<dfn export for=CSS id="dfn-support-selector">support a CSS selector</dfn>
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if it accepts that selector
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(rather than discarding it as a parse error),
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and that selector doesn't contain
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<a>unknown -webkit- pseudo-elements</a>.
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<div class="example">
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In this example, three different color font technologies
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are tested, in order of preference,
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plus a monochrome fallback.
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The most capable, COLRv1, supports both gradients and font variations;
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the next best choice, SVG, supports gradients
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while the least capable, COLRv0, supports flat color fill only.
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The fallback has no test condition,
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so will always be chosen unless one of the earlier conditions succeeds.
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<pre class="lang-css">
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@when font-tech(color-COLRv1) and font-tech(variations) {
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@font-face { font-family: icons; src: url(icons-gradient-var.woff2); }
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}
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@else font-tech(color-SVG) {
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@font-face { font-family: icons; src: url(icons-gradient.woff2); }
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}
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@else font-tech(color-COLRv0) {
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@font-face { font-family: icons; src: url(icons-flat.woff2); }
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}
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@else {
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@font-face { font-family: icons; src: url(icons-fallback.woff2); }
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}
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</pre>
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Notice that in this example,
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the variable color font is only downloaded
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if COLRv1 is supported
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and font variations are also supported.
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Notice too that only one of the available options will be downloaded;
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this would not be the case without ''@when'' and ''@else'',
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as the next example shows.
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</div>
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<h2 class=no-num id="security">Security Considerations</h2>
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<div class="example">
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In this example,
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although it appears that the fallback will not be used
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if COLRv1 is supported,
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in fact both fonts will be downloaded,
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which wastes bandwidth if it is not used.
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No Security issues have been raised against this document
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The fallback might still be used for some characters;
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for example, if the color font supports only Latin,
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while the fallback supports Latin and Greek.
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<h2 class=no-num id="privacy">Privacy Considerations</h2>
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<pre class="lang-css">
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@font-face { font-family: icons; src: url(icons-fallback.woff2);
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@supports font-tech(color-COLRv1) {
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@font-face { font-family: icons; src: url(icons-gradient-var.woff2); }
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}
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</pre>
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<!-- COPY FROM L3 when importing the rest of the spec -->
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</div>
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The ''selector()'' function may provide information about the user's software
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such as its version
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and whether it is running with non-default settings that enable or disable certain features.
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<h2 class=no-num id="security">Security Considerations</h2>
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This information can also be determined through other APIs.
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However, the features in this specification are one of the ways this information
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is exposed on the Web.
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No Security issues have been raised against this document
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This information can also, in aggregate, be used to improve the accuracy of
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<a href="https://www.w3.org/2001/tag/doc/unsanctioned-tracking/">fingerprinting</a> of the user.
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<h2 class=no-num id="privacy">Privacy Considerations</h2>
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<h2 class=no-num id="acknowledgments">Acknowledgments</h2>
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No Privacy issues have been raised against this document
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<!-- Firstly, the -->The editors would like to thank
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all of the contributors to the <a href="http://www.w3.org/TR/css-values-3/#acknowledgements">previous level</a>
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of this module.
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<!-- Secondly, we would like to acknowledge -->
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<h2 class=no-num id="acknowledgments">Acknowledgments</h2>
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<h2 id='changes' class='no-num'>
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Changes</h2>
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The @when and @else rules are based on a proposal by Tab Atkins.
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<h3 id="changes-from-20200303" class="no-num">
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Changes since the <a href="https://www.w3.org/TR/2020/WD-css-conditional-4-20200303/">First Public Working Draft of 3 March 2020</a></h3>
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<h2 id='changes' class='no-num'>
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Changes</h2>
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<ul>
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<li>Added Privacy and Security sections.
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</ul>
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<h3 id="changes-from-20200303" class="no-num">Changes since the
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<a href="https://www.w3.org/TR/2020/WD-css-conditional-4-20200303/">First Public Working Draft of 3 March 2020</a>
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</h3>
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<h3 id="changes-from-L4" class="no-num">
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Additions since Level 3</h3>
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<ul>
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<li>Renamed font-technology to font-tech</li>
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<li>Added some examples</li>
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<li>Added @when and @else per CSSWG resolution</li>
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<li>Extended @supports feature to express font capabilities</li>
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<li>Added two co-editors per CSSWG resolution</li>
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<li>Added Privacy and Security sections</li>
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</ul>
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<ul>
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<li>Added ''selector()'' notation to [=supports queries=].
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</ul>

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