[css-anchor-position-1] Change position-anchor initial value to 'auto', and only let you specify that behavior in anchor()/anchor-size() via omission. w3c#10312

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -402,8 +402,8 @@ Default Anchors: the 'position-anchor' property</h3>
 
 <pre class=propdef>
 Name: position-anchor
-Value: <<anchor-element>>
-Initial: implicit
+Value: auto | <<anchor-element>>
+Initial: auto
 Applies to: [=absolutely positioned=] elements
 Inherited: no
 Animation type: discrete
@@ -421,7 +421,10 @@ The [=target anchor element=] selected by the [=default anchor specifier=]
 is the element's <dfn>default anchor element</dfn>.
 
 Its values are identical to the <<anchor-element>> term
-in ''anchor()'' and ''anchor-size()''.
+in ''anchor()'' and ''anchor-size()'',
+except that the ''position-anchor/auto'' value
+is equivalent to omitting the <<anchor-element>> in ''anchor()''
+(referring to the [=implicit anchor element=], if one exists).
 
 <div class=example>
 	For example, in the following code
@@ -706,7 +709,7 @@ It is only valid to be used in the [=inset properties=].
 
 <pre class=prod>
 	&lt;anchor()> = anchor( <<anchor-element>>? <<anchor-side>>, <<length-percentage>>? )
-	<dfn><<anchor-element>></dfn> = <<dashed-ident>> | implicit
+	<dfn><<anchor-element>></dfn> = <<dashed-ident>>
 	<dfn><<anchor-side>></dfn> = inside | outside
 			   | top | left | right | bottom
 			   | start | end | self-start | self-end
@@ -718,15 +721,14 @@ The ''anchor()'' function has three arguments:
 * the <<anchor-element>> value
 	specifies how to find the [=anchor element=]
 	it will be drawing positioning information from.
-	If omitted, it behaves as the element's [=default anchor specifier=].
 	Its possible values are:
 
 	<dl dfn-type=value dfn-for="anchor()">
 		: <dfn><<dashed-ident>></dfn>
 		:: Specifies the [=anchor name=] it will look for.
 			This name is a [=tree-scoped reference=].
 
-		: <dfn>implicit</dfn>
+		: omitted
 		:: Selects the [=implicit anchor element=]
 			defined for the element,
 			if possible.