Make [CEReactions]'s treatment of exceptions explicit

Fixes #3217.

Author: domenic

source

@@ -66945,19 +66945,28 @@ customElements.define("x-foo", class extends HTMLElement {
   Additionally, it must not appear on readonly attributes.</p>
 
   <p>Operations, attributes, setters, or deleters annotated with the <code
-  data-x="CEReactions">[CEReactions]</code> extended attribute must run the following steps
-  surrounding the actions listed in the description of the operation, setter, deleter, or the
-  attribute's setter:</p>
+  data-x="CEReactions">[CEReactions]</code> extended attribute must run the following steps in place
+  of the ones specified in their description:</p>
 
-  <dl>
-   <dt>Before executing the listed actions</dt>
-   <dd>Push a new <span>element queue</span> onto the <span>custom element reactions
-   stack</span>.</dd>
+  <ol>
+   <li><p><span data-x="stack push">Push</span> a new <span>element queue</span> onto the
+   <span>custom element reactions stack</span>.</p></li>
 
-   <dt>After executing the listed actions</dt>
-   <dd>Pop the <span>element queue</span> from the <span>custom element reactions stack</span>, and
-   <span>invoke custom element reactions</span> in that queue.</dd>
-  </dl>
+   <li><p>Run the originally-specified steps for this construct, catching any exceptions. If the
+   steps return a value, let <var>value</var> be the returned value. If they throw an exception, let
+   <var>exception</var> be the thrown exception.</p></li>
+
+   <li><p>Let <var>queue</var> be the result of <span data-x="stack pop">popping</span> from the
+   <span>custom element reactions stack</span>.</p></li>
+
+   <li><p><span>Invoke custom element reactions</span> in <var>queue</var>.</p></li>
+
+   <li><p>If an exception <var>exception</var> was thrown by the original steps, rethrow
+   <var>exception</var>.</p></li>
+
+   <li><p>If a value <var>value</var> was returned from the original steps, return
+   <var>value</var>.</p></li>
+  </ol>
 
   <div class="note">
    <p>The intent behind this extended attribute is somewhat subtle. One way of accomplishing its