[css-anchor-position] Make anchor-size()'s axis keyword optional. #10318

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1244,7 +1244,7 @@ The ''anchor-size()'' Function {#anchor-size-fn}
 ------------------------------
 
 <pre class=prod>
-anchor-size() = anchor-size( <<anchor-element>>? && <<anchor-size>>, <<length-percentage>>? )
+anchor-size() = anchor-size( [ <<anchor-element>> || <<anchor-size>> ]? , <<length-percentage>>? )
 <dfn><<anchor-size>></dfn> = width | height | block | inline | self-block | self-inline
 </pre>
 
@@ -1273,6 +1273,12 @@ according to either the [=writing mode=] of the element
 or the [=writing mode=] of the element's [=containing block=]
 (for ''anchor-size()/block'' and ''anchor-size()/inline'').
 
+If the <<anchor-size>> keyword is omitted,
+it defaults to behaving as whatever keyword
+matches the axis of the property that ''anchor-size()'' is used in.
+(For example, ''width: anchor-size()''
+is equivalent to ''width: anchor-size(width)''.)
+
 An ''anchor-size()'' function representing a [=valid anchor-size function=]
 resolves at [=computed value=] time
 (via [=style & layout interleaving=])