[css-pseudo-4] Define that pseudo(type) does a CSS-parse of the argument, and clarify the wording around always returning the same object. Fixes #3603.

Author: tabatkins

css-pseudo-4/Overview.bs

@@ -16,6 +16,7 @@ Editor: Alan Stearns, Adobe Systems Inc., stearns@adobe.com, w3cid 46659
 Abstract: This CSS module defines pseudo-elements, abstract elements that represent portions of the CSS render tree that can be selected and styled.
 Ignored Terms: initial-letter, PseudoElement, pseudo(), selectors
 Default Highlight: css
+Indent: 2
 </pre>
 <pre class="link-defaults">
 spec:css-color-3; type:value; text:currentcolor
@@ -890,16 +891,32 @@ Additions to the CSS Object Model</h2>
     };
   </pre>
 
-  The <dfn method for=Element title="pseudo(type)" id="dom-element-pseudo">pseudo(CSSOMString type)</dfn> method
-  is used to retrieve the {{CSSPseudoElement}} instance
-  of the type matching {{type!!argument}}
-  associated with the element.
-  Its return value on a given element
-  for a given, supported {{CSSPseudoElement}} {{CSSPseudoElement/type}} is,
-  insofar as observable, always the same {{CSSPseudoElement}} object.
+  <div algorithm>
+    The <dfn method for=Element title="pseudo(type)" id="dom-element-pseudo">pseudo(CSSOMString type)</dfn> method
+    is used to retrieve the {{CSSPseudoElement}} instance
+    of the type matching {{type!!argument}}
+    associated with the element.
+    When it is called,
+    execute the following steps:
+
+    1. [=CSS/Parse=] the {{type!!argument}} argument as a <<pseudo-element-selector>>,
+      and let |type| be the result.
+
+    2. If |type| is failure,
+      return null.
+
+    3. Otherwise, return the {{CSSPseudoElement}} object
+      representing the pseudo-element
+      that would match the selector |type|
+      with [=this=] as its [=originating element=].
+  </div>
+
+  Return values that represent the same [=pseudo-element=]
+  on the same [=originating element=]
+  must be, insofar as observable,
+  always the same {{CSSPseudoElement}} object.
   (The UA may drop or regenerate the object for convenience or performance
   if this is not observable.)
-  For unrecognized types, the return value is null.
 
   ISSUE: The identity, lifetime, and nullness of the return value
   (and potential error cases)