[css-values-5] Loosen the syntax of attr-unit so unknown units trigger fallback rather than being invalid. #13060

Author: tabatkins

css-values-5/Overview.bs

@@ -1883,6 +1883,7 @@ Ian's proposal:
 
 		<dfn>&lt;attr-name></dfn> = [ <<ident-token>>? '|' ]? <<ident-token>>
 		<dfn>&lt;attr-type></dfn> = <dfn function lt="type()" for="attr()">type( <<syntax>> )</dfn> | raw-string | number | <<attr-unit>>
+		<dfn>&lt;attr-unit></dfn> = <<custom-ident>>
 	</pre>
 
 	The ''attr()'' function's [=argument grammar=] is:
@@ -1898,13 +1899,15 @@ Ian's proposal:
 	Note: That is, ''attr(foo)'' does not pass a fallback value,
 	but ''attr(foo,)'' does (the fallback is just empty).
 
-	The <dfn>&lt;attr-unit></dfn> production matches any [=identifier=]
+	<<attr-unit>> is intended to represents an [=identifier=]
 	that is an [=ASCII case-insensitive=] match
 	for the name of a CSS dimension unit,
 	such as ''px'',
 	or the <<delim-token>> <css>%</css>.
-	It is not expanded literally here,
-	as the set of units expands over time.
+	As this set expands regularly,
+	it is actually specified as a <<custom-ident>>,
+	and triggers fallback behavior when it doesn't match a known unit
+	(rather than making the function invalid).
 
 	<!-- Switch the <attr-name> to just use <q-name>
 		when Namespaces is rewritten
@@ -1957,13 +1960,16 @@ Ian's proposal:
 		Values that fail to parse trigger fallback.
 
 		If given as an <<attr-unit>> value,
-		the value is first parsed as if ''number'' keyword was specified,
-		then the resulting numeric value
-		is turned into a dimension with the corresponding unit,
-		or a percentage if <css>%</css> was given.
-		Same as for ''number'' <<attr-type>>, 
-		values that do not correspond to the <<number-token>> production
-		trigger fallback.
+		the value is first parsed as if ''number'' keyword was specified;
+		if this fails to parse,
+		it triggers fallback.
+		Then, if the <<attr-unit>> value
+		matches a known CSS unit name
+		or is <css>%</css>,
+		the number is turned into a dimension or percentage
+		with that value and the given unit.
+		If the <<attr-unit>> does not match a known CSS unit,
+		it triggers fallback.
 
 		If given as the <css>raw-string</css> keyword,
 		or omitted entirely,
@@ -3081,7 +3087,7 @@ Tree Counting Functions: the ''sibling-count()'' and ''sibling-index()'' notatio
 
 
 	A [=tree-counting function=]
-	is a type of  [=tree-scoped-reference/loosely-matched=] [=tree-scoped reference=],
+	is a type of  [=tree-scoped reference/loosely-matched=] [=tree-scoped reference=],
 	which is resolved as if the element were given an automatic [=tree-scoped name=]
 	that matches an identical automatic name on the [=tree-scoped reference=]
 	of the [=tree-counting function=].