[css-pseudo] Extend CSSPseudoElement to all standardized non-element-backed tree-abiding pseudo-elements (w3c#13940)

* [css-pseudo] Extend CSSPseudoElement to all standardized non-element-backed tree-abiding pseudo-elements

* Reject non-tree-abiding pseudos

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: danielsakhapov

css-pseudo-4/Overview.bs

@@ -1810,7 +1810,9 @@ Additions to the CSS Object Model</h2>
 
   The <dfn attribute for=CSSPseudoElement>type</dfn> attribute
   is a string representing the type of the pseudo-element.
-  This can be one of the following values:
+  This can be any standardized [=tree-abiding pseudo-element=]
+  that is not an [=element-backed pseudo-element=],
+  for example:
 
   <dl>
     <dt><code>"::before"</code>
@@ -1819,6 +1821,10 @@ Additions to the CSS Object Model</h2>
     <dd>Represents the ''::after'' pseudo-element.
     <dt><code>"::marker"</code>
     <dd>Represents the ''::marker'' pseudo-element.
+    <dt><code>"::backdrop"</code>
+    <dd>Represents the ''::backdrop'' pseudo-element.
+    <dt><code>"::view-transition"</code>
+    <dd>Represents the ''::view-transition'' pseudo-element.
   </dl>
 
   The <dfn attribute for=CSSPseudoElement>element</dfn> attribute is the
@@ -1881,7 +1887,13 @@ Additions to the CSS Object Model</h2>
     2. If |type| is failure,
       return null.
 
-    3. Otherwise, return the {{CSSPseudoElement}} object
+    3. If |type| is an [=element-backed pseudo-element=],
+      return null.
+
+    4. If |type| is not a [=tree-abiding pseudo-element=],
+      return null.
+
+    5. Otherwise, return the {{CSSPseudoElement}} object
       representing the pseudo-element
       that would match the selector |type|
       with [=this=] as its [=originating element=].