Add number keyword for attr() and specify how <attr-unit> should be parsed (#12862)

* Add number keyword for attr() and specify how <attr-unit> should be parsed

* Update css-values-5/Overview.bs

* Update css-values-5/Overview.bs

---------

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: tursunova

css-values-5/Overview.bs

@@ -1867,7 +1867,7 @@ Ian's proposal:
 		attr() = attr( <<attr-name>> <<attr-type>>? , <<declaration-value>>?)
 
 		<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 | <<attr-unit>>
+		<dfn>&lt;attr-type></dfn> = <dfn function lt="type()" for="attr()">type( <<syntax>> )</dfn> | raw-string | number | <<attr-unit>>
 	</pre>
 
 	The ''attr()'' function's [=argument grammar=] is:
@@ -1929,12 +1929,19 @@ Ian's proposal:
 		Values that fail to parse according to the syntax
 		trigger fallback.
 
+		If given as the <css>number</css> keyword,
+		it causes the attribute's literal value,
+		after [=strip leading and trailing ASCII whitespace|stripping leading and trailing whitespace=],
+		to be [=CSS/parsed=] as a <<number-token>>.
+		Values that fail to parse trigger fallback.
+
 		If given as an <<attr-unit>> value,
-		the value is first parsed as if <css>type(&lt;number>)</css> was specified,
+		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.
-		Values that fail to parse as a <css>&lt;number></css>
+		Same as for ''number'' <<attr-type>>, 
+		values that do not correspond to the <<number-token>> production
 		trigger fallback.
 
 		If given as the <css>raw-string</css> keyword,
@@ -2040,20 +2047,27 @@ Substitution</h4>
 			let |attr value| be its value;
 			otherwise jump to the last step (labeled FAILURE).
 
-		4. If |syntax| is null
+		4. If |syntax| is the keyword ''number'' or an <<attr-unit>> value,
+			parse |attr value| against <<attr-type>>.
+			If that succeeds, return the result;
+			otherwise, jump to the last step (labeled FAILURE).
+
+			Note: No parsing or modification of any kind is performed on the value.
+		
+		5. If |syntax| is null
 			or the keyword ''raw-string'',
 			return a CSS <<string>>
 			whose value is |attr value|.
 
 			Note: No parsing or modification of any kind is performed on the value.
 
-		5. [=Substitute arbitrary substitution functions=] in |attr value|,
+		6. [=Substitute arbitrary substitution functions=] in |attr value|,
 			with &bs<<;"attribute", |attr name|&bs>>; as the [=substitution context=],
 			then [=parse with a <syntax>=] |attr value|, with |syntax| and |el|.
 			If that succeeds, return the result;
 			otherwise, jump to the last step (labeled FAILURE).
 
-		6. <b>FAILURE:</b>
+		7. <b>FAILURE:</b>
 
 			1. If |second arg| is null,
 				and |syntax| was omitted,