[css-values-4] Add definition of grammar production blocks. #2921

Author: fantasai

css-values-4/Overview.bs

@@ -479,6 +479,41 @@ Property Value Examples</h3>
 		</table>
 	</div>
 
+<h3 id="production-blocks">
+Non-Terminal Definitions and Grammar Production Blocks</h3>
+
+	The precise grammar of non-terminals, like <<position>> or <<calc()>>,
+	is often specified in a <dfn export>CSS grammar production block</dfn>.
+	These are conventionally represented in a preformatted block of definitions
+	like this:
+
+	<div class="example">
+		The <css>&lt;foo></css> syntax is defined as follows:
+
+		<xmp class=prod>
+			<foo> = keyword | <bar> |
+			        some-really-long-pattern-of-stuff
+			<bar> = <length>
+		</xmp>
+	</div>
+
+	Each definition starts on its own line,
+	and consists of the non-terminal to be defined,
+	followed by an <code>=</code>,
+	followed by the fragment of [=value definition syntax=] to which it expands.
+	A definition can stretch across multiple lines,
+	and terminates before the next line that starts a new grammar production
+	or at the end of the grammar production block
+	(whichever comes first).
+
+	<div class="example">
+		In the above example, the <css>&lt;foo></css> definition covers two lines.
+		The third line starts a new definition for <css>&lt;bar></css>.
+		(A naked <code>=</code> is never valid in [=value definition syntax=],
+		so it's unambiguous when a new line starts a fresh definition.)
+	</div>
+
+
 <h2 id="combining-values">
 Combining Values: Interpolation, Addition, and Accumulation</h2>
 
@@ -5196,6 +5231,8 @@ Recent Changes</h3>
 		despite violation of the “avoid dataloss by default principle”--
 		given existing interoperability and presumed Web-compat restriction.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/6454">Issue 6452</a>)
+	* Added an explicit definition for the [=CSS grammar production block=] convention.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/2921">Issue 2921</a>)
 
 	Substantial changes since <a href="https://www.w3.org/TR/2023/WD-css-values-4-20230406/">6 April 2023 WD</a>: