[css-syntax] Add a hook for parsing something according to a CSS grammar.

--HG--
extra : rebase_source : 1417377dc483c4f3e4d906bf5cb936f5879693be

Author: tabatkins

css-syntax/Overview.bs

@@ -2100,6 +2100,50 @@ Parser Entry Points</h3>
 	All of the algorithms defined in this spec may be called with either a list of tokens or of component values.
 	Either way produces an identical result.
 
+<h4 id="parse-grammar">
+Parse something according to a CSS grammar</h4>
+
+	It is often desirable to parse a string or token list
+	to see if it matches some CSS grammar,
+	and if it does,
+	to destructure it according to the grammar.
+	This section provides a generic hook for this kind of operation.
+	It should be invoked like "parse <var>foo</var> as a CSS <<color>>", or similar.
+
+	Note: As a reminder, this algorithm, along with all the others in this section,
+	can be called with a string,
+	a stream of CSS tokens,
+	or a stream of CSS component values,
+	whichever is most convenient.
+
+	This algorithm must be called with <b>some input to be parsed</b>,
+	and <b>some CSS grammar specification or term</b>.
+
+	This algorithm returns either failure,
+	if the input does not match the provided grammar,
+	or the result of parsing the input according to the grammar,
+	which is an unspecified structure corresponding to the provided grammar specification.
+	The return value must only be interacted with by specification prose,
+	where the representation ambiguity is not problematic
+	if it is meant to be exposed outside of spec language,
+	the spec using the result must explicitly translate it into a well-specified representation,
+	such as, for example, by invoking a CSS serialization algorithm
+	(like "serialize as a CSS <<string>> value").
+
+	To <dfn export>parse something according to a CSS grammar</dfn>:
+
+	<ol>
+		<li>
+			<a>Parse a list of component values</a> from the input,
+			and let <var>result</var> be the return value.
+
+		<li>
+			Attempt to match <var>result</var> against the provided grammar.
+			If this is successful,
+			return the matched result;
+			otherwise, return failure.
+	</ol>
+
 
 <h4 id="parse-a-stylesheet">
 Parse a stylesheet</h4>