[css-anchor-position-1] Add position-anchor:none and make it the initial value. Fix some markup. w3c#13067

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -500,8 +500,8 @@ Default Anchors: the 'position-anchor' property</h3>
 
 <pre class=propdef>
 Name: position-anchor
-Value: auto | <<anchor-name>>
-Initial: auto
+Value: none | auto | <<anchor-name>>
+Initial: none
 Applies to: [=absolutely positioned boxes=]
 Inherited: no
 Animation type: discrete
@@ -513,6 +513,9 @@ and (by default) all [=anchor functions=] applied to this element.
 'position-anchor' is a [=reset-only sub-property=] of 'position'.
 
 <dl dfn-for=position-anchor dfn-type=value>
+	: <dfn>none</dfn>
+	::
+		The box has no [=default anchor element=].
 
 	: <dfn>auto</dfn>
 	::
@@ -890,6 +893,7 @@ The ''anchor()'' function has three arguments:
 			and ''100%'' being equivalent to ''anchor()/end''.
 
 			''anchor()/center'' is equivalent to ''50%''.
+	</dl>
 
 * the optional <<length-percentage>> final argument is a fallback value,
 	specifying what the function should compute to
@@ -2605,7 +2609,7 @@ Significant changes since the <a href="https://www.w3.org/TR/2025/WD-css-anchor-
 * Define ''anchor-center'' to also use the [=scrollable containing block=]
 	so that it doesn't trigger overflow alignment when positioned outside the [=local containing block=].
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12952">Issue 12952</a>)
-* Resolve ''margin/auto'' margins to zero when 'position-area' or 'anchor-center' is in effect,
+* Resolve ''margin/auto'' margins to zero when 'position-area' or ''anchor-center'' is in effect,
 	due to the ill-considered HTML UA default stylesheet rules for popovers.
 	Also drop the <css>dialog</css> alignment value which was the previous attempt to address this problem.
 	(<a href="https://github.com/w3c/csswg-drafts/issues/10258">Issue 10258</a>)