[css-values-4] Note-ify comment about url() vs src() parsing.

Also improve cross-referencing to css-syntax.

Author: fantasai

css-values-4/Overview.bs

@@ -932,13 +932,15 @@ Resource Locators: the <<url>> type</h3>
 		</pre>
 	</div>
 
-	For legacy reasons, a ''url()'' can be written without quotation marks around the URL itself,
+	A ''url()'' can be written without quotation marks around the URL value,
 	in which case it is <a lt="consume a url token" spec=css-syntax-3>specially-parsed</a>
-	as a <<url-token>> [[!CSS-SYNTAX-3]].
-	Because of this special parsing, ''url()'' is only able to specify its URL literally;
-	''src()'' lacks this special parsing rule,
-	and so its URL can be provided by functions,
-	such as ''var()''.
+	as a <<url-token>>; see [[css-syntax-3#consume-url-token]]. [[!CSS-SYNTAX-3]]
+
+	Note: Because of this special parsing,
+	''url()'' can only express its value literally.
+	To provide a URL by functions such as ''var()'',
+	use the ''src()'' notation,
+	which does not have this special parsing rule.
 
 	<div class="example">
 		For example, the following declarations are identical:
@@ -967,23 +969,18 @@ Resource Locators: the <<url>> type</h3>
 		so the entire declaration is thrown out as invalid.
 	</div>
 
-	<div class=note>
-		The unquoted ''url()'' syntax cannot accept a <<url-modifier>> argument
-		and has extra escaping requirements:
-		parentheses, <a href="https://www.w3.org/TR/css-syntax/#whitespace">whitespace</a> characters,
-		single quotes (&#39;) and double quotes (") appearing in a URL
-		must be escaped with a backslash,
-		e.g. ''url(open\(parens)'', ''url(close\)parens)''.
-		(In quoted <<string>> ''url()''s,
-		only newlines and the character used to quote the string need to be escaped.)
-		Depending on the type of URL,
-		it might also be possible to write these characters as URL-escapes
-		(e.g. ''url(open%28parens)'' or ''url(close%29parens)'')
-		as described in [[URL]].
-
-		The precise requirements for parsing the unquoted ''url()'' syntax
-		are normatively defined in [[!CSS-SYNTAX-3]].
-	</div>
+	Note: The unquoted ''url()'' syntax cannot accept a <<url-modifier>> argument
+	and has extra escaping requirements:
+	parentheses, <a href="https://www.w3.org/TR/css-syntax/#whitespace">whitespace</a> characters,
+	single quotes (&#39;) and double quotes (") appearing in a URL
+	must be escaped with a backslash,
+	e.g. ''url(open\(parens)'', ''url(close\)parens)''.
+	(In quoted <<string>> ''url()''s,
+	only newlines and the character used to quote the string need to be escaped.)
+	Depending on the type of URL,
+	it might also be possible to write these characters as URL-escapes
+	(e.g. ''url(open%28parens)'' or ''url(close%29parens)'')
+	as described in [[URL]].
 
 	Some CSS contexts (such as ''@import'') also allow a <<url>>
 	to be represented by a bare <<string>>, without the function wrapper.