[cssom] Throw a TypeError when the pseudo-element passed to getComputedStyle is unknown or ::slotted.

Fixes #2149

Author: emilio

cssom-1/Overview.bs

@@ -2769,16 +2769,18 @@ steps:
 <ol>
  <li>Let <var>doc</var> be <var>elt</var>'s <a>node document</a>.
  <li>Let <var>obj</var> be <var>elt</var>.
- <li>If <var>pseudoElt</var> is as an
- <a>ASCII case-insensitive</a> match for
- either ":before" or "::before" let
- <var>obj</var> be the ''::before'' pseudo-element of
- <var>elt</var>.
- <li>If <var>pseudoElt</var> is as an
- <a>ASCII case-insensitive</a> match for
- either ":after" or "::after" let
- <var>obj</var> be the ''::after'' pseudo-element of
- <var>elt</var>.
+ <li>If <var>pseudoElt</var> is provided, and it's not the empty string, then:
+  <ol>
+    <li>If it is an <a>ASCII case-insensitive</a> match for a pseudo-element
+    that the UA supports, other than ''::slotted()'', let <var>obj</var>
+    be the given pseudo-element of <var>elt</var>.
+    Otherwise <a>throw</a> a {{TypeError}} exception.
+  </ol>
+
+  Note: CSS2 pseudo-elements should match both the double and single-colon
+  versions. That is, both <code>:before</code> and <code>::before</code> should
+  match above.
+
  <li>
   Return a live <a>CSS declaration block</a> with the following properties:
   <dl>