[css-values-4] Fully omitting a mix() argument is fine (you don't need to use whitespace). w3c#8556

Author: tabatkins

css-values-4/Overview.bs

@@ -548,7 +548,7 @@ Representing Interpolated Values: the ''mix()'' notation</h3>
 	whose syntax is defined as follows:
 
 	<pre class="prod">
-		&lt;mix()> = mix( <<percentage>> ';' <<declaration-value>> ';' <<declaration-value>> )
+		&lt;mix()> = mix( <<percentage>> ';' <<declaration-value>>? ';' <<declaration-value>>? )
 	</pre>
 
 	<dl dfn-for="mix()">
@@ -565,6 +565,13 @@ Representing Interpolated Values: the ''mix()'' notation</h3>
 			The second <<declaration-value>> represents
 			the value at the “end” (100%) of the interpolation range,
 			interpreted the same.
+
+			Either value can be omitted,
+			to refer to an empty value
+			(this is valid in [=custom properties=], such as ''--foo:;'').
+			The separating '';'' cannot be omitted;
+			for example, a ''mix()'' with an empty starting value
+			would be written like ''mix(50%;;bar)''.
 	</dl>
 
 	Note: This [=functional notation=] uses semicolons to separate arguments
@@ -5180,6 +5187,8 @@ Recent Changes</h3>
 			(<a href="https://github.com/w3c/csswg-drafts/issues/8527">Issue 8527</a>)
 		<li>Specified that negative <<resolution>> values are out-of-range by definition.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/8532">Issue 8532</a>)
+		<li>Clarified that fully omitted ''mix()'' arguments are valid.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/8556">Issue 8556</a>)
 	</ul>
 
 	Substantial changes since <a href="https://www.w3.org/TR/2021/WD-css-values-4-20211016/">16 October 2021 WD</a>: