[css-syntax] Define <any-value> and <declaration-value>.

Author: tabatkins

css-syntax/Overview.bs

@@ -3139,6 +3139,37 @@ Defining Block Contents: the <<declaration-list>>, <<rule-list>>, and <<styleshe
 		which causes them to be dropped from the outer rule's value if they appear.
 	</div>
 
+<h3 id="any-value">
+Defining Arbitrary Contents: the <<declaration-value>> and <<any-value>> productions</h3>
+
+	In some grammars,
+	it is useful to accept any reasonable input in the grammar,
+	and do more specific error-handling on the contents manually
+	(rather than simply invalidating the construct,
+	as grammar mismatches tend to do).
+
+	For example, <a>custom properties</a> allow any reasonable value,
+	as they can contain arbitrary pieces of other CSS properties,
+	or be used for things that aren't part of existing CSS at all.
+	For another example, the <<general-enclosed>> production in Media Queries
+	defines the bounds of what future syntax MQs will allow,
+	and uses special logic to deal with "unknown" values.
+
+	To aid in this, two additional productions are defined:
+
+	The <dfn>&lt;declaration-value></dfn> production matches <em>any</em> sequence of one or more tokens,
+	so long as the sequence does not contain
+	<<bad-string-token>>,
+	<<bad-url-token>>,
+	unmatched <<)-token>>, <<]-token>>, or <<}-token>>,
+	or top-level <<semicolon-token>> tokens or <<delim-token>> tokens with a value of "!".
+	It represents the entirety of what a valid declaration can have as its value.
+
+	The <dfn>&lt;any-value></dfn> production is identical to <<declaration-value>>,
+	but also allows top-level <<semicolon-token>> tokens
+	and <<delim-token>> tokens with a value of "!".
+	It represents the entirety of what valid CSS can be in any context.
+
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