[selectors-4] Specify forgiving-selector parsing, and use it in :is() and :where(). (Per WG resolution, we dont' use it in :not().) #3264

Author: tabatkins

selectors-4/Overview.bs

@@ -996,11 +996,6 @@ Invalid Selectors and Error Handling</h3>
 		<li>an empty selector, i.e. one that contains no <a>compound selector</a>, is invalid.
 	</ul>
 
-	ISSUE: The CSSWG indends to modify these rules such that an invalid selector
-	in the selector list argument of a pseudo-class like `:is()`
-	is simply ignored rather than invalidating the whole selector list.
-	See <a href="https://github.com/w3c/csswg-drafts/issues/3264">Issue 3264</a>.
-
 	Note: Consistent with CSS’s forwards-compatible parsing principle,
 	UAs <em>must</em> treat as <a>invalid</a>
 	any pseudo-classes, pseudo-elements, combinators, or other syntactic constructs
@@ -1066,8 +1061,12 @@ Selector Lists</h3>
 The Matches-Any Pseudo-class: '':is()''</h3>
 
 	The matches-any pseudo-class, <dfn id='matches-pseudo'>:is()</dfn>,
-	is a functional pseudo-class taking a <a>selector list</a> as its argument.
-	It represents an element that is represented by its argument.
+	is a functional pseudo-class taking a <<forgiving-selector-list>> as its sole argument.
+
+	If the argument, after parsing, is an empty list,
+	the pseudo-class is valid but matches nothing.
+	Otherwise, the pseudo-class matches any element
+	that matches any of the selectors in the list.
 
 	Note: The specificity of the '':is()'' pseudo-class
 	is replaced by the specificity of its most specific argument.
@@ -1165,7 +1164,7 @@ The Specificity-adjustment Pseudo-class: '':where()''</h3>
 	The Specificity-adjustment pseudo-class, <dfn id="where-pseudo">:where()</dfn>,
 	is a functional pseudo-class
 	with the same syntax and functionality as '':is()''.
-	Unlike '':is()'', neither the '':where'' pseudo-class, nor any of its arguments
+	Unlike '':is()'', neither the '':where()'' pseudo-class, nor any of its arguments,
 	contribute to the specificity of the selector--
 	its specificity is always zero.
 
@@ -1209,7 +1208,7 @@ The Specificity-adjustment Pseudo-class: '':where()''</h3>
 The Relational Pseudo-class: '':has()''</h3>
 
 	The relational pseudo-class, <dfn id='has-pseudo'>:has()</dfn>,
-	is a functional pseudo-class taking a <a>relative selector</a> <a lt="selector list">list</a> as an argument.
+	is a functional pseudo-class taking a <<forgiving-relative-selector-list>> as an argument.
 	It represents an element if any of the <a>relative selectors</a>,
 	when <a href="#absolutize">absolutized</a>
 	and evaluated with the element as the <a>:scope elements</a>,
@@ -3476,8 +3475,52 @@ Grammar</h2>
 	but these are specified on a case-by-case basis.
 	(For example, the <a>user action pseudo-classes</a> are allowed after any <a>pseudo-element</a>,
 	and the <a>tree-abiding pseudo-elements</a>
-	are allowed after the ''::slotted()'' pseudo-element)
+	are allowed after the ''::slotted()'' pseudo-element.)
+
+<h3 id='forgiving-selector'>
+<<forgiving-selector-list>> and <<forgiving-relative-selector-list>></h3>
+
+	For legacy reasons,
+	the general behavior of a selector list
+	is that if any selector in the list fails to parse
+	(because it uses new or UA-specific selector features, for instance),
+	the entire selector list becomes invalid.
+	This can make it hard to write CSS that uses new selectors
+	and still works correctly in older user agents.
+
+	The <dfn><<forgiving-selector-list>></dfn> production
+	instead parses each selector in the list individually,
+	simply ignoring ones that fail to parse,
+	so the remaining selectors can still be used.
+
+	Note: Style rules still use the normal, unforgiving selector list behavior.
+	<<forgiving-selector-list>> is used in some functions,
+	like '':is()'',
+	which are similarly generic.
+	Although it does have some minor implications on specificity,
+	simply wrapping a style rule's selector in '':is()''
+	effectively "upgrades" it to become forgiving.
+
+	Syntactically, <<forgiving-selector-list>> is equivalent to <code><<any-value>>?</code>.
+	It is then [=parsed as a forgiving selector list=]
+	to obtain its actual value.
+
+	<div algorithm>
+		To <dfn export lt="parse as a forgiving selector list|parsed as a forgiving selector list|parsing as a forgiving selector list">parse as a forgiving selector list</dfn> given an input |input|:
+
+		1. [=Parse a list=] of <<complex-selector>>s from |input|,
+			and let |selector list| be the result.
+
+		3. Remove all failure items from |selector list|,
+			and all items that are [=invalid selectors=],
+			then return a <<selector-list>>
+			representing the remaining items in |selector list|.
+			(This might be empty.)
+	</div>
 
+	<dfn><<forgiving-relative-selector-list>></dfn> is identical to <<forgiving-selector-list>>,
+	except it parses its components as <<relative-selector>>
+	rather than <<complex-selector>>.
 
 <h2 id='api-hooks'>
 API Hooks</h2>