CSS Conditional Rules Module Level 5

1. Introduction

This is currently an early draft of the things that are new in level 5. The features in Level 3 and Level 4 are still defined in [css-conditional-3] and [css-conditional-4] and have not yet been copied here.

CSS Conditional Level 5 extends the @supports rule and supports query syntax to allow testing for supported font technologies.

It also adds an @when rule, which generalizes the concept of a conditional rule. Anything that can be expressed in an existing conditional rule can be expressed in @when by wrapping it in an appropriate function to declare what kind of condition it is. This allow authors to easily combine multiple types of queries, such as media queries and supports queries, in a single boolean expression. Without this, authors must rely on nesting separate conditional rules, which is harder to read and write, presupposes the conditions are to be conjoined with the “and” boolean relation (with no easy way to indicate anything else), and restricts their utility in the proposed conditional rule chains.

It also adds @else rules, which immediately follow other conditional rules and automatically qualify their conditions as the inverse of the immediately preceding rule’s conditions, such that only the first matching rule in a conditional rule chain is applied.

It also adds Container Queries. They are conceptually similar to Media Queries, but allow testing aspects of elements within the document (such as box dimensions or computed styles), rather than on the document as a whole.

2. Extensions to the @supports rule

This level of the specification extends the <supports-feature> syntax as follows:

<supports-feature> = <supports-selector-fn> | <supports-font-tech-fn>
          | <supports-font-format-fn> | <supports-decl>
<supports-font-tech-fn> = font-tech( <font-tech> )
<supports-font-format-fn> = font-format( <font-format> )

<supports-font-tech-fn>

The result is true if the UA supports the font tech provided as an argument to the function.

<supports-font-format-fn>

The result is true if the UA supports the font format provided as an argument to the function.

2.1. Extensions to the definition of support

A CSS processor is considered to support a font tech when it is capable of utilizing the specified CSS Fonts 4 § 11.1 Font tech in layout and rendering.

A CSS processor is considered to support a font format when it is capable of utilizing the specified CSS Fonts 4 § 11.2 Font formats in layout and rendering, and this format is not specified as a <string>.

3. Generalized Conditional Rules: the @when rule

The @when at-rule is a conditional group rule that generalizes the individual conditional group rules such as @media and @supports. It is defined as:

@when <boolean-condition> {
  <rule-list>
}

Where <boolean-condition> is a boolean algebra a la Media Queries 4 § 3 Syntax, but with media() and supports() functions as leaves.

Define "boolean algebra, with X as leaves" in a generic way in Conditional, so all the conditional rules can reference it directly, rather than having to redefine boolean algebra on their own.

The media() and supports() functions are defined as:

media() = media( [ <mf-plain> | <mf-boolean> | <mf-range> ] )
supports() = supports( <declaration> )

A media() or supports() function is associated the boolean result that its contained condition is associated with.

4. Chained Conditionals: the @else rule

Usually, conditional group rules are independent; each one has a separate condition evaluated without direct reference to any other rule, and decides whether or not to apply its contained rules based solely on its condition.

This is fine for simple conditions, but makes it difficult to write a collection of conditionals that are meant to be mutually exclusive: authors have to very carefully craft their conditions to not activate when the other rules are meant to, and make sure the collection of conditionals don’t accidentally all exclude some situation which is then left unstyled.

The @else rule is a conditional group rule used to form conditional rule chains, which associate multiple conditional group rules and guarantee that only the first one that matches will evaluate its condition as true. It is defined as:

@else <boolean-condition>? {
  <rule-list>
}

@else is interpreted identically to @when. If its <boolean-condition> is omitted, it’s treated as having a condition that’s always true.

A conditional rule chain is a series of consecutive conditional group rules, starting with a conditional group rule other than @else, followed by zero or more @else rules. There cannot be anything between the successive conditional group rules other than whitespace and/or comments; any other token “breaks” the chain.

Should we require that only the last @else in a chain can have an omitted condition? It’s not uncommon for me, when debugging code, to short-circuit an if-else chain by setting one of them to "true"; I presume that would be similarly useful in CSS? It’s still pretty easy to see you’ve done something wrong if you omit the condition accidentally.

Within a conditional rule chain, the conditions of each conditional group rule are evaluated in order. If one of them is true, the conditions of all following conditional group rules in the chain evaluate to false, regardless of their stated condition.

An @else rule that is not part of a conditional rule chain is invalid and must be ignored.

@when media(width >= 400px) and media(pointer: fine) and supports(display: flex) {
  /* A */
} @else supports(caret-color: pink) and supports(background: double-rainbow()) {
  /* B */
} @else {
  /* C */
}

@media (width >= 400px) and (pointer: fine) {
  @supports (display: flex) {
  /* A */
  }
  @supports not (display: flex) {
  @supports (caret-color: pink) and (background: double-rainbow()) {
    /* B */
  }
  @supports not ((caret-color: pink) and (background: double-rainbow())) {
    /* C */
  }
  }
}
@media not ((width >= 400px) and (pointer: fine)) {
  @supports (caret-color: pink) and (background: double-rainbow()) {
  /* B */
  }
  @supports not ((caret-color: pink) and (background: double-rainbow())) {
  /* C */
  }
}

@when font-tech(color-COLRv1) and font-tech(variations) {
  @font-face { font-family: icons; src: url(icons-gradient-var.woff2); }
}
@else font-tech(color-SVG) {
  @font-face { font-family: icons; src: url(icons-gradient.woff2); }
}
@else font-tech(color-COLRv0) {
  @font-face { font-family: icons; src: url(icons-flat.woff2); }
}
@else {
  @font-face { font-family: icons; src: url(icons-fallback.woff2); }
}

@font-face { font-family: icons; src: url(icons-fallback.woff2);
@supports font-tech(color-COLRv1) {
  @font-face { font-family: icons; src: url(icons-gradient-var.woff2); }
}

5. Container Queries

While media queries provide a method to query aspects of the user agent or device environment that a document is being displayed in (such as viewport dimensions or user preferences), container queries allow testing aspects of elements within the document (such as box dimensions or computed styles).

By default, all elements are query containers for the purpose of container style queries, and can be established as query containers for container size queries and container scroll-state queries by specifying the additional query types using the container-type property (or the container shorthand). Style rules applying to a query container’s flat tree descendants can be conditioned by querying against it, using the @container conditional group rule.

main, aside {
  container: my-layout / inline-size;
}

.media-object {
  display: grid;
  grid-template: 'img' auto 'content' auto / 100%;
}

@container my-layout (inline-size > 45em) {
  .media-object {
  grid-template: 'img content' auto / auto 1fr;
  }
}

For the ::part() and ::slotted() pseudo-element selectors, which represent real elements in the DOM tree, query containers can be established by flat tree ancestors of those elements. For other pseudo-elements, query containers can be established by inclusive flat tree ancestors of their originating element.

<style>
  #container {
  width: 100px;
  container-type: inline-size;
  }
  @container (inline-size < 150px) {
  #inner::before {
    content: "BEFORE";
  }
  }
</style>
<div id=container>
  <span id=inner></span>
</div>

<div id=host style="width:200px">
  <template shadowroot=open>
  <style>
    #container {
    width: 100px;
    container-type: inline-size;
    }
    @container (inline-size < 150px) {
    ::slotted(span) {
      color: green;
    }
    }
  </style>
  <div id=container>
    <slot />
  </div>
  </template>
  <span id=slotted>Green</span>
</div>

5.1. Creating Query Containers: the container-type property

The container-type property establishes the element as a query container for certain types of queries. For size container queries, which require certain types of containment, elements are explicitly made query containers through this property. For other types of query containers any element can be a query container, such as for container style queries.

Values have the following meanings:

size

Establishes a query container for container size queries on both the inline and block axis. Applies style containment and size containment to the principal box, and establishes an independent formatting context.

inline-size

Establishes a query container for container size queries on the container’s own inline axis. Applies style containment and inline-size containment to the principal box, and establishes an independent formatting context.

scroll-state

Establishes a query container for container scroll-state queries

normal

The element is not a query container for any container size queries or container scroll-state queries, but remains a query container for container style queries.

aside, main {
  container-type: inline-size;
}

h2 { font-size: 1.2em; }

@container (width > 40em) {
  h2 { font-size: 1.5em; }
}

@container style(--cards: small) {
  article {
  border: thin solid silver;
  border-radius: 0.5em;
  padding: 1em;
  }
}

#sticky {
  container-type: scroll-state;
  position: sticky;
}
@container scroll-state(stuck: top) {
  #sticky-child {
  background-color: lime;
  }
}

5.2. Naming Query Containers: the container-name property

The container-name property specifies a list of query container names. These names can be used by @container rules to filter which query containers are targeted. Container names are not tree-scoped names.

none

The query container has no query container name.

<custom-ident>

Specifies a query container name as an identifier. The keywords none, and, not, and or are excluded from this <custom-ident>.

main {
  container-type: size;
  container-name: my-page-layout;
}

.my-component {
  container-type: inline-size;
  container-name: my-component-library;
}

@container my-page-layout (block-size > 12em) {
  .card { margin-block: 2em; }
}

@container my-component-library (inline-size > 30em) {
  .card { margin-inline: 2em; }
}

@container my-page-layout {
  .card { padding: 1em; }
}

5.3. Creating Named Containers: the container shorthand