[css-cascade-4] Legacy name aliases require exact match between value syntaxes. #4839

Author: fantasai

css-cascade-4/Overview.bs

@@ -331,8 +331,8 @@ Aliasing</h3>
 	<dl export>
 		<dt><dfn lt="legacy name alias">legacy name aliases</dfn>
 		<dd>
-			When the old property’s syntax is identical to
-			or a subset of the value space of the new property’s syntax,
+			When the old property’s value syntax is identical
+			to that of the new property,
 			the two names are aliased with an operation on par with case-mapping:
 			at parse time, the old property is converted into the new property.
 			This conversion also applies in the CSSOM,
@@ -1172,7 +1172,16 @@ Changes</h2>
 <h3 id="changes-2018">
 Changes Since the 28 August 2018 Candidate Recommendation</h3>
 
-	Non-trivial changes since the <a href="https://www.w3.org/TR/2018/CR-css-cascade-4-20180828/">28 August 2018 Working Draft</a> include:
+	Non-trivial changes since the <a href="https://www.w3.org/TR/2020/WD-css-cascade-4-20200818/">18 August 2020 Working Draft</a> include:
+
+	<ul class=non-normative>
+		<li id="change-2020-alias-subset">
+			Removed possibility of [=legacy name aliases=] to map subsets of the value space,
+			since they are simple name aliases.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/4839">Issue 4839</a>)
+	</ul>
+
+	Non-trivial changes since the <a href="https://www.w3.org/TR/2018/CR-css-cascade-4-20180828/">28 August 2018 Candidate Recommendation</a> include:
 
 	<ul class=non-normative>
 		<li id="change-2018-context">