CSS Route Matching

1. Declaring named URL patterns: the @route rule

The @route rule is an at-rule that associates a name with a URL pattern. This name can be referenced in @navigation rules and in :link-to() pseudo-classes.

The syntax of the @route rule is:

@route <dashed-ident> {
  [ <pattern-descriptors> | <init-descriptors> ]
}

based on the following definitions:

<pattern-descriptors> = ;* <pattern-descriptor> ;*
<pattern-descriptor> = pattern : <url-pattern()>
<init-descriptors> = ;* <init-descriptor> [ ;+ <init-descriptor> ]* ;*
<init-descriptor> = <init-descriptor-name> : <string>
<init-descriptor-name> = protocol | username | password | hostname | port
                         pathname | search | hash | base-url

This associates an author-defined keyword with a URL pattern, so that any URL that matches one of the URL patterns matches the route named by the keyword.

The @route rule can be defined in one of two ways:

with the pattern descriptor

in this case the URL pattern represented is the one represented by the <url-pattern()> function given as the descriptor’s value.

with the other descriptors named by <init-descriptor-name>

In this case the URL pattern represented is the result of invoking create a URL pattern given input as URLPatternInit constructed from the descriptors and their values. Each dictionary member is given the value of the descriptor with the same name, except the baseURL member is given the value of the base-url descriptor. If a base-url descriptor is not given then one is created from the style resource base URL of the rule.

Should this use <dashed-ident>, <custom-ident>, or <ident> for the route names?

Is there value in being able to provide a list of <url-pattern()> rather than just one?

@route --movie-list {
  pattern: url-pattern("/movie-list");
}

@route --movie-list {
  pathname: "/movie-list";
}

NOTE: The bracing syntax also allows for future expansion if needed.

2. Conditional rules for navigation queries

2.1. Navigation queries: the @navigation rule

The @navigation rule is a conditional group rule whose condition tests characteristics of the current URL or of the state of navigation between two URLs. These queries are called navigation queries.

Authors can use it to:

• write style sheets that apply to multiple pages but behave somewhat differently between those pages,

• write style sheets that apply to single page applications that change their URL over time, so that style changes when the URL changes, and

• write style sheets that declaratively start view transitions (or make other appropriate style changes) in response to navigations.

The syntax of the condition in the @navigation rule is similar to that defined for <supports-condition> in [CSS-CONDITIONAL-3]. Negation, conjunction, and disjunction are all needed so that authors can specify the interaction of multiple styles in ways that are most intuitive and require the simplest code.

@navigation (at: url-pattern("/")) {
  /* These styles only apply to the site’s homepage
     (including any URL with a search or hash). */
}

@route --search-results-page {
  pattern: url-pattern("/search-results");
}
@route --product-page {
  pattern: url-pattern("/product/:id");
}

@navigation ((from: --search-results-page) and
             (to: --product-page)) or
            ((from: --product-page) and
             (to: --search-results-page)) {
  /* These styles apply when a navigation is in progress
     between a search results page and a product page (as
     defined by the @route rules above), in either
     direction. */
}

The syntax of the @navigation rule is:

@navigation <navigation-condition> {
  <rule-list>
}

with <navigation-condition> defined as:

<navigation-condition> = not <navigation-in-parens>
                     | <navigation-in-parens> [ and <navigation-in-parens> ]*
                     | <navigation-in-parens> [ or <navigation-in-parens> ]*
<navigation-in-parens> = ( <navigation-condition> ) | ( <navigation-test> ) | <general-enclosed>
<navigation-test> = <navigation-location> | <navigation-keyword> : <navigation-location>
<navigation-keyword> = at | from | to
<navigation-location> = <route-name> | <url-pattern()>
<route-name> = <dashed-ident>

The above grammar is purposely very loose for forwards-compatibility reasons, since the <general-enclosed> production allows for substantial future extensibility. Any @navigation 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).

Many of these grammar terms are associated with a boolean result, as follows:

<navigation-condition>

not <navigation-in-parens>

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

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

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

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

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

<navigation-in-parens>

The result is the result of the child subexpression.

<navigation-test>

<navigation-location>

at: <navigation-location>

The result is whether the result of match a URL pattern is non-null given urlPattern as the navigation location URL pattern of <navigation-location> and input as the document’s URL.

from: <navigation-location>

The result is true if the current from URL from of the document is non-null and match a URL pattern is non-null when given urlPattern as the navigation location URL pattern of <navigation-location> and input as from.

to: <navigation-location>

The result is true if the current to URL to of the document is non-null and match a URL pattern is non-null when given urlPattern as the navigation location URL pattern of <navigation-location> and input as to.

<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 <navigation-condition> in older user agents.

The navigation location URL pattern of a <navigation-location> depends on the type of <navigation-location>:

<route-name>

the URL pattern represented by the @route rule referenced by the name.

<url-pattern()>

The URL pattern represented by the function; see create a URL pattern for url-pattern().

Should it also be possible to reference a name defined in a routemap? See the route matching explainer for details.

A document’s navigation API is the result of the following steps on document:

1. Let window be the Window whose associated Document is document, or null if there is no such Window.

2. If window is null, return null.

3. Return window’s navigation API.

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

2.2. The navigation() function for @when

This specification defines an additional function for the @when rule:

navigation() = navigation( <navigation-condition> )

The navigation() function is associated with the boolean result that its contained condition is associated with.

2.3. The navigation() function for if()

This specification defines an additional function for the if() function’s <if-test> production:

navigation() = navigation( <navigation-condition> )

This should probably have a more formal definition of the function, but I can’t find the formal definitions of the existing if() functions to model it after.

3. Pseudo-class for navigation-related links: :link-to()

This specification defines a new :link-to() functional pseudo-class that matches link elements that link to a certain URL.

:link-to(url-pattern("/")) {
  font-weight: bold;
}

@view-transition {
  /* allow cross-document view transitions */
  navigation: auto;
}

@route --movie-details {
  /* match URLs like /en/movie/123 which is the English page
     about a movie with ID 123 */
  pattern: url-pattern("/:lang/movie/:id");
}

/* capture the overall area representing the movie, and a
   sub-area for its poster image */

/* match an element with class movie-container with a child
   link that links to a movie whose id is the same as the
   movie we are currently navigating to or from.  (lang can
   be different, though.)

   Just :link-to(--movie-details) requires that the target
   of the link match the URL pattern defined by the "@route
   --movie-details" rule.

   The navigation-param(id) further requires that either the
   from or the to URL of the current navigation also match
   the URL pattern represented by the "@route
   --movie-details" rule, and that that the 'id' named group
   from that match be the same as the 'id' named group from
   the match with the link’s target.
   */
.movie-container:has(> .movie-title:link-to(
  --movie-details with navigation-param(id))) {

  view-transition-name: movie-container;

  > .movie-poster {
    view-transition-name: movie-poster;
  }

  /* leave the default cross-fade animation for both image
     captures */
}

The :link-to() pseudo-class takes a single argument, a <link-condition>, and the pseudo-class matches any element where:

• the element matches :any-link

• the target of link matches the <link-condition>, as defined below.

<link-condition> = <link-condition-base> [ with <navigation-param-expression> ]?
<link-condition-base> = <navigation-location>
<navigation-param-expression> = ( <navigation-param-and> ) |
                                ( <navigation-param-or> ) |
                                <navigation-param>
<navigation-param-and> = <navigation-param-expression>
                         [ "and" <navigation-param-expression> ]*
<navigation-param-or> = <navigation-param-expression>
                        [ "or" <navigation-param-expression> ]*
<navigation-param> = <navigation-param-key-value> | <navigation-param-function>
<navigation-param-key-value> = ( <ident> : <string> )
<navigation-param-function> = navigation-param( <ident> )

A <link-condition> matches the target of the link when both:

• the <link-condition-base> matches the target of the link, and

• the <navigation-param-expression> matches the target of the link, with the URL pattern represented by the <link-condition-base> as context

A <link-condition-base> represents a URL pattern. If the <link-condition-base> is a <url-pattern()>, then it represents the URL pattern represented by the <url-pattern()> function (see create a URL pattern for url-pattern()). If it is a <route-name>, then it represents the URL pattern represented by the @route rule.

A <link-condition-base> matches a URL when match a URL pattern is non-null given urlPattern as the URL pattern that it represents and input as the given URL.

A <navigation-param-expression> matches a URL (with a URL Pattern as context) based on standard boolean logic for and and or, and based on whether each <navigation-param> matches the URL (with the URL Pattern as context).

A <navigation-param> matches the URL input (with a URL Pattern urlPattern as context) if the following steps return true:

1. Let matchResult be the result of match a URL pattern given urlPattern and input. the URL pattern represented by the <url-pattern()> function in the <link-condition-base> (see create a URL pattern for url-pattern())

2. If matchResult is null, return false.

   NOTE: This doesn’t really matter because in this case the <link-condition-base> also doesn’t match.

3. For each property prop of matchResult (a URLPatternResult) that is a URLPatternComponentResult:

   1. For each entry entry in its groups, then:

      1. If <navigation-param> is a <navigation-param-key-value>, return true if entry’s key is the <ident> and entry’s value is the <string>.

      2. If <navigation-param> is a <navigation-param-function>, and entry’s key is the function’s <ident>, then for each navigationUrl of the current to URL and current from URL:

         1. Let navigationMatchResult be the result of match a URL pattern given urlPattern and navigationUrl.

         2. If navigationMatchResult is null, continue.

         3. Return true if navigationMatchResult’s property prop has an entry whose key is the same as entry’s key and whose value is the same as entry’s value.

            NOTE: This step makes the navigation-param() function perform what is essentially a three-way match, between the target of the link, the provided URL pattern, and the from or to URL of the current navigation.

4. Current navigation URLs

Both the @navigation rule and the :link-to() pseudo-class rely on the following definitions of the current from URL and current to URL.

The current from URL of a document is a URL or null. It is defined as follows:

1. If the document’s navigation API of the document is non-null and its transition is non-null, its from entry’s url.

   NOTE: This part is for when the old document in the navigation is still the current document.

2. If the document’s navigation API of the document is non-null and its activation is non-null, the document’s has been revealed is false or was false at the start of the current task, the activation’s from’s url.

   NOTE: This part is for when the new document in the navigation has become the current document.

3. Otherwise, null.

   NOTE: The previous two branches can also produce null results.

The current to URL of a document is a URL or null. It is defined as follows:

1. If the document’s navigation API of the document is non-null and its ongoing navigate event is non-null:

   1. if the pageswap event has fired since that navigation began, and its activation was non-null, and that activation’s entry’s url is non-null, then that url.

      NOTE: This part does expose the result of redirects.

      Is the final "non-null" check needed?

   2. otherwise, the ongoing navigate event’s destination’s url

      NOTE: This part does not expose the result of redirects.

   This assumes that the ongoing navigate event and the transition have the same lifetime, but this isn’t really true if the event is intercepted. After whatwg/html#11690 / whatwg/html#11692. we could probably define this more like "from" above. But which lifetime is the one we want?

   NOTE: This part is for when the old document in the navigation is still the current document.

2. If the document’s navigation API of the document is non-null and its activation is non-null, the document’s has been revealed is false or was false at the start of the current task, and the activation’s entry’s url.

   NOTE: This part is for when the new document in the navigation has become the current document.

3. Otherwise, null.

   NOTE: The previous two branches can also produce null results.

The above definitions of from and to apparently don’t work right if you start a same-document navigation (e.g., with pushState) in the middle of a cross-document navigation.

Generally improve integration with the HTML spec for these definitions, instead of monkeypatching. This includes the interaction with has been revealed and the interaction with the pageswap event, and other things where this section links to non-exported definitions.

5. The url-pattern() function

The url-pattern() function represents a URL pattern, which can be used to match URLs.

<url-pattern()> = url-pattern( <string> )

This function represents a URL pattern that can be created using the steps of the create a URL pattern for url-pattern() algorithm:

1. Let arg be the <string> argument to the url-pattern() function.

2. Let baseURL be the style resource base URL of the rule or declaration block containing the url-pattern() function.

   Do we want this to be the base URL all the time? For use of url-pattern() in @navigation, it’s likely more useful for the base URL to be the document URL rather than the style sheet URL. However, it would be very awkward for url-pattern() to be inconsistent with url().

   Should we allow the base URL of url-pattern() to be defined by the consumer? Should we introduce document-url-pattern()? Should we do something similar to CSS Images 3 § 2.1.1 Ambiguous Reference-or-Image URLs (see w3c/csswg-drafts issue #383)?

   Also see other proposed uses of URLPattern in CSS in w3c/csswg-drafts issue #10975, for :local-link.

3. Return the result of create a URL pattern given arg, baseURL, and an empty map.

NOTE: This function requires that its argument is quoted. This differs from the url() function, which allows its argument to be quoted or unquoted.

To serialize a url-pattern() function f, serialize a function f, using serialize a string on the single argument to serialize f’s contents.

NOTE: This is defined this way because URLPattern intentionally does not provide a serialization.