[css-values] Per f2f discussion, commas are now implicitly omittable in some circumstances. Nor more contortions to handle separating commas, woo\!

Author: tabatkins

css-values/Overview.bs

@@ -100,6 +100,46 @@ Component value types</h3>
 	such as “+”,
 	must be written enclosed in single quotes.
 
+	Commas specified in the grammar are <strong>implicitly omittable</strong> in some circumstances,
+	when used to separate optional terms in the grammar.
+	Within a top-level list in a property or other CSS value,
+	or a function's argument list,
+	a comma specified in the grammar must be omitted if:
+
+	<ul>
+		<li>
+			all items preceding the comma have been omitted
+
+		<li>
+			all items following the comma have been omitted
+
+		<li>
+			multiple commas would be adjacent (ignoring whitespace/comments),
+			due to the items between the commas being omitted.
+	</ul>
+
+	<div class='example'>
+		For example, if a function can accept three arguments in order,
+		but all of them are optional,
+		the grammar can be written like:
+
+		<pre class='prod'>
+		example( first? , second? , third? )
+		</pre>
+
+		Given this grammar,
+		writing ''example(first, second, third)'' is valid,
+		as is ''example(first, second)'' or ''example(first, third)'' or ''example(second)''.
+		However, ''example(first, , third)'' is invalid, as one of those commas are no longer separating two options;
+		similarly, ''example(,second)'' and ''example(first,)'' are invalid.
+		''example(first second)'' is also invalid,
+		as commas are still required to actually separate the options.
+
+		If commas were not implicitly omittable,
+		the grammar would have to be much more complicated
+		to properly express the ways that the arguments can be omitted,
+		greatly obscuring the simplicity of the feature.
+	</div>
 
 	All CSS properties also accept the <a href="#common-keywords">CSS-wide keyword values</a>
 	as the sole component of their property value.