[css-anchor-position-1] Compensate for stupid UA default rules by resolving margins to auto #10258

Also drop the 'dialog' alignment value, which is no longer needed.

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -651,7 +651,9 @@ Values other than ''position-area/none'' have the following additional effects:
 	when the [=absolute-position containing block=] is generated by a [=scroll container=],
 	so that the entire [=scrollable overflow area=] (typically) is available
 	for positioning.
-* Any ''top/auto'' [=inset properties=] resolve to ''0''.
+* The [=used value=] of any ''top/auto'' [=inset properties=]
+	and ''margin/auto'' [=margin properties=]
+	resolves to ''0''.
 * The ''align-self/normal'' value for the [=self-alignment properties=]
 	resolves to a corresponding value, see [[#position-area-alignment]].
 
@@ -1317,7 +1319,9 @@ Additionally:
 	where applicable,
 	so that the entire [=scrollable overflow area=] (typically) is available
 	for positioning.
-* Any ''top/auto'' [=inset properties=] resolve to ''0''.
+* The [=used value=] of any ''top/auto'' [=inset properties=]
+	and ''margin/auto'' [=margin properties=]
+	resolves to ''0''.
 
 If the box is not [=absolutely positioned=],
 or does not have a [=default anchor box=],
@@ -1331,38 +1335,6 @@ it will “shift” from being purely centered,
 in order to remain within the [=original containing block=].
 See [[css-align-3#overflow-values]] for more details.
 
-<!-- Big Text: dialog
-
-████▌  ████  ███▌  █▌     ███▌   ███▌
-█▌  █▌  ▐▌  ▐█ ▐█  █▌    █▌  █▌ █▌  █▌
-█▌  █▌  ▐▌  █▌  █▌ █▌    █▌  █▌ █▌
-█▌  █▌  ▐▌  █▌  █▌ █▌    █▌  █▌ █▌ ██▌
-█▌  █▌  ▐▌  █████▌ █▌    █▌  █▌ █▌  █▌
-█▌  █▌  ▐▌  █▌  █▌ █▌    █▌  █▌ █▌  █▌
-████▌  ████ █▌  █▌ █████  ███▌   ███▌
--->
-
-Conditional Centering: the ''dialog'' alignment value {#conditional-centering}
------------------------------------------------------
-
-<pre class=propdef>
-Name: justify-self, align-self, justify-items, align-items
-New Values: dialog
-</pre>
-
-Dialog boxes are often displayed centered on the screen,
-unless they are anchored to some other element.
-
-The new <dfn value for="justify-self, align-self, justify-items, align-items">dialog</dfn> value
-allows this behavior to be specified:
-if the box is [=absolutely positioned=]
-and its 'position-area' value is not ''position-area/none'',
-it is aligned as per ''align-self/normal''
-(see [[#position-area-alignment]]);
-otherwise,
-it is aligned as per ''align-self/center''.
-
-
 <!-- Big Text: a-size()
 
  ███▌         ███▌  ████ █████▌ █████▌   ██ ██  
@@ -2633,6 +2605,10 @@ 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,
+	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>)
 * Clarify that ''flip-block'', ''flip-inline'', and ''flip-start'' use the [=writing mode=] of the [=containing block=].
 	(<a href="https://github.com/w3c/csswg-drafts/issues/12869">Issue 12869</a>)
 * Add 'clip-path' to the list of clipping effects considered for ''anchors-visible''