[css-properties-values-api] Require <custom-ident> as component name. (#884)

* [css-properties-values-api] Require <custom-ident> as component name.

Something like this was in the original syntax parsing PR, but it got
lost at some point.

Resolves #879.

* fixup! [css-properties-values-api] Require <custom-ident> as component name.

* Update css-properties-values-api/Overview.bs

Co-Authored-By: andruud <andruud@gmail.com>


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

Author: andruud

css-properties-values-api/Overview.bs

@@ -640,6 +640,9 @@ Parsing the syntax string {#parsing-syntax}
 		and set |component|’s |name| to the returned value.
 		Otherwise return failure.
 
+		If |component|’s |name| does not [=CSS/parse=] as a <<custom-ident>>,
+		return failure.
+
 	:   anything else
 	::  Return failure.