Overview.bs

<pre class="metadata">
Title: CSS Route Matching
Status: ED
Work Status: Exploring
Shortname: css-navigation
Level: 1
Group: csswg
ED: https://drafts.csswg.org/css-navigation-1/
!Issue Tracking: <a href="https://github.com/w3c/csswg-drafts/issues/12594">w3c/csswg-drafts#12594</a>
Editor: L. David Baron, Google https://www.google.com/, https://dbaron.org/, w3cid 15393
Editor: Noam Rosenthal, Google https://www.google.com/, w3cid 121539
Abstract: This module contains conditional CSS rules for styling conditioned on the current URL
        or conditioned on the status of navigating between particular URLs.
</pre>

<pre class="link-defaults">
spec:css-values-5; type:function; text:if()
</pre>

<!-- FIXME: TEMPORARILY override non-exported definition -->
<pre class=anchors>
url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#concept-navigationtransition-from
	type: dfn; spec: html; text: from entry;
url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#window-navigation-api
	type: dfn; spec: html; text: navigation API;
url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#ongoing-navigate-event
	type: dfn; spec: html; text: ongoing navigate event;
url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#concept-navigation-transition
	type: dfn; spec: html; text: transition;
url: https://html.spec.whatwg.org/multipage/nav-history-apis.html#navigation-activation
	type: dfn; spec: html; text: activation;
url: https://html.spec.whatwg.org/multipage/browsing-the-web.html#has-been-revealed
	type: dfn; spec: html; text: has been revealed;
</pre>

<h2 id="condional-navigation-queries">Conditional rules for navigation queries</h2>

<h3 id="at-navigation">Navigation queries: the ''@navigation'' rule</h3>

The <dfn at-rule id="at-ruledef-navigation">@navigation</dfn> 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 <dfn export>navigation queries</dfn>.

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.

The syntax of the ''@navigation'' rule is:

<pre class="prod def" nohighlight>
@navigation <<navigation-condition>> {
	<<rule-list>>
}
</pre>

with <<navigation-condition>> defined as:

<pre class="prod def" dfn-type="type" nohighlight>
	<dfn><<navigation-condition>></dfn> = not <<navigation-in-parens>>
	                     | <<navigation-in-parens>> [ and <<navigation-in-parens>> ]*
	                     | <<navigation-in-parens>> [ or <<navigation-in-parens>> ]*
	<dfn><<navigation-in-parens>></dfn> = ( <<navigation-condition>> ) | ( <<navigation-test>> ) | <<general-enclosed>>
	<dfn><<navigation-test>></dfn> = <<navigation-location>> | <<navigation-keyword>> : <<navigation-location>>
	<dfn><<navigation-keyword>></dfn> = at | from | to
	<dfn><<navigation-location>></dfn> = <<urlpattern()>>
</pre>

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 <strong>must not</strong> use such a rule and
processors <strong>must</strong> 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 true if
		the document's [=Document/URL=] [=URL pattern/match|matches=]
		the [=navigation location URL pattern=] of <<navigation-location>>.

	: from: <<navigation-location>>
	:: The result is true if
		the [=document's navigation API=] of the document
		is non-null, and either:

		* its [=transition=] is non-null,
			its [=from entry=]'s {{NavigationHistoryEntry/url}}
			is non-null and
			[=URL pattern/match|matches=]
			the [=navigation location URL pattern=] of <<navigation-location>>.

		* 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 {{NavigationActivation/from}}'s
			{{NavigationHistoryEntry/url}}'s
			is non-null and
			[=URL pattern/match|matches=]
			the [=navigation location URL pattern=] of <<navigation-location>>.

	: to: <<navigation-location>>
	:: The result is true if
		the [=document's navigation API=] of the document
		is non-null, and either:

		* its [=ongoing navigate event=] is non-null,
			and its {{NavigateEvent/destination}}'s
			{{NavigationDestination/url}}
			[=URL pattern/match|matches=]
			the [=navigation location URL pattern=] of <<navigation-location>>.

			ISSUE: 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
			<a href="https://github.com/whatwg/html/issues/11690">whatwg/html#11690</a> /
			<a href="https://github.com/whatwg/html/pull/11692">whatwg/html#11692</a>.
			we could probably define this more like "from" above.
			But which lifetime is the one we want?

		* 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 {{NavigationActivation/entry}}'s
			{{NavigationHistoryEntry/url}}'s
			is non-null and
			[=URL pattern/match|matches=]
			the [=navigation location URL pattern=] of <<navigation-location>>.

		TODO: Update this definition to handle redirects better.
		In particular, we'd like the ''to:'' URL to change
		at only a single time,
		from the original URL of the navigation to the final one.
		There is already a well-defined definition and processing model
		that we can depend on.

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

	ISSUE: Improve integration with [=has been revealed=] rather than monkeypatching it.

: <<general-enclosed>>
::
	The result is false.

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

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

: <<urlpattern()>>
::	The [=URL pattern=] represented by the function; see
	[=create a URL pattern for urlpattern()=].

ISSUE: There is currently only one type of <<navigation-location>>
but we should consider adding another,
which would be just a name that references
a name defined in a routemap.
See the
<a href="https://github.com/WICG/declarative-partial-updates/blob/main/route-matching-explainer.md">route matching explainer</a>
for details.

A <dfn>document's navigation API</dfn> is
the result of the following steps on <var>document</var>:

1. Let <var>window</var> be the {{Window}} whose [=associated Document=] is <var>document</var>, or null if there is no such {{Window}}.

1. If <var>window</var> is null, return null.

1. Return <var>window</var>'s [=navigation API=].

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

<h3 id="navigation-when-function">The ''@when/navigation()'' function for ''@when''</h3>

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

<pre class="prod">
<dfn for="@when" function>navigation()</dfn> = navigation( <<navigation-condition>> )
</pre>

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

<h3 id="navigation-if-function">The ''if()/navigation()'' function for ''if()''</h3>

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

<pre class="prod">
<dfn for="if()" function>navigation()</dfn> = navigation( <<navigation-condition>> )
</pre>

ISSUE: 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.

<h2 id="link-navigation-pseudo-classes">Pseudo-classes for navigation-related links</h2>

TODO: To be written.

<h2 id="urlpattern-function">The ''urlpattern()'' function</h2>

<!--

NOTE:  We may eventually want to move this to css-values.

If we do, the definition of "style resource base URL" probably doesn't need to be
exported any more, since it was exported for this definition.

-->

The <dfn export function>urlpattern()</dfn> function represents a [=URL pattern=],
which can be used to match URLs.

<pre class="prod def">
<<urlpattern()>> = urlpattern( <<string>> )
</pre>

This function represents a [=URL pattern=] that can be created
using the steps of the <dfn>create a URL pattern for urlpattern()</dfn> algorithm:

1. Let <var>arg</var> be the <<string>> argument to the ''urlpattern()'' function.

1. Let <var>baseURL</var> be the [=style resource base URL=] of
	the rule or declaration block containing the ''urlpattern()'' function.

	<div class="issue">
	Do we want this to be the base URL all the time?
	For use of ''urlpattern()'' 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 ''urlpattern()''
	to be inconsistent with ''url()''.

	Should we allow the base URL of ''urlpattern()''
	to be defined by the consumer?
	Should we introduce <code>document-urlpattern()</code>?
	Should we do something similar to
	[[css-images-3#ambiguous-urls]]
	(see <w3c/csswg-drafts#383>)?

	Also see other proposed uses of {{URLPattern}} in CSS
	in <w3c/csswg-drafts#10975>,
	for '':local-link''.
	</div>

1. Return the result of [=URL pattern/create|create a URL pattern=] given
	<var>arg</var>, <var>baseURL</var>, 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 <dfn export>serialize a ''urlpattern()'' function</dfn> <var>f</var>,
[=serialize a function=] <var>f</var>,
using [=serialize a string=] on the single argument
to serialize <var>f</var>'s contents.

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