Ground custom elements in agents

The "unit of related similar-origin browsing contexts" concept is inaccurate.

Helps with #4198.

source

@@ -67050,12 +67050,12 @@ customElements.define("x-foo", class extends HTMLElement {
 
   <hr>
 
-  <p>Each <span>unit of related similar-origin browsing contexts</span> has a <dfn>custom element
-  reactions stack</dfn>, which is initially empty. The <dfn>current element queue</dfn> is the
-  <span>element queue</span> at the top of the <span>custom element reactions stack</span>. Each
-  item in the stack is an <dfn>element queue</dfn>, which is initially empty as well. Each item in
-  an <span>element queue</span> is an element. (The elements are not necessarily <span
-  data-x="concept-element-custom">custom</span> yet, since this queue is used for <span
+  <p>Each <span>similar-origin window agent</span> has a <dfn>custom element reactions stack</dfn>,
+  which is initially empty. A <span>similar-origin window agent</span>'s <dfn>current element
+  queue</dfn> is the <span>element queue</span> at the top of its <span>custom element reactions
+  stack</span>. Each item in the stack is an <dfn>element queue</dfn>, which is initially empty as
+  well. Each item in an <span>element queue</span> is an element. (The elements are not necessarily
+  <span data-x="concept-element-custom">custom</span> yet, since this queue is used for <span
   data-x="custom-element-upgrades">upgrades</span> as well.)</p>
 
   <p>Each <span>custom element reactions stack</span> has an associated <dfn>backup element
@@ -67088,31 +67088,38 @@ customElements.define("x-foo", class extends HTMLElement {
   <var>element</var>, run the following steps:</p>
 
   <ol>
+   <li><p>Let <var>reactionsStack</var> be <var>element</var>'s <span>relevant agent</span>'s
+   <span>custom element reactions stack</span>.</p></li>
+
    <li>
-    <p>If the <span>custom element reactions stack</span> is empty, then:</p>
+    <p>If <var>reactionsStack</var> is empty, then:</p>
 
     <ol>
-     <li><p>Add <var>element</var> to the <span>backup element queue</span>.</p></li>
+     <li><p>Add <var>element</var> to <var>reactionsStack</var>'s <span>backup element
+     queue</span>.</p></li>
 
-     <li><p>If the <span>processing the backup element queue</span> flag is set, then
-     return.</p></li>
+     <li><p>If <var>reactionsStack</var>'s <span>processing the backup element queue</span> flag is
+     set, then return.</p></li>
 
-     <li><p>Set the <span>processing the backup element queue</span> flag.</p></li>
+     <li><p>Set <var>reactionsStack</var>'s <span>processing the backup element queue</span>
+     flag.</p></li>
 
      <li>
       <p><span>Queue a microtask</span> to perform the following steps:</p>
 
       <ol>
-       <li><p><span>Invoke custom element reactions</span> in the <span>backup element
-       queue</span>.</p></li>
+       <li><p><span>Invoke custom element reactions</span> in <var>reactionsStack</var>'s
+       <span>backup element queue</span>.</p></li>
 
-       <li><p>Unset the <span>processing the backup element queue</span> flag.</p></li>
+       <li><p>Unset <var>reactionsStack</var>'s <span>processing the backup element queue</span>
+       flag.</p></li>
       </ol>
      </li>
     </ol>
    </li>
 
-   <li><p>Otherwise, add <var>element</var> to the <span>current element queue</span>.</p></li>
+   <li><p>Otherwise, add <var>element</var> to <var>element</var>'s <span>relevant agent</span>'s
+   <span>current element queue</span>.</p></li>
   </ol>
 
   <p>To <dfn data-export="">enqueue a custom element callback reaction</dfn>, given a <span>custom
@@ -67218,15 +67225,15 @@ customElements.define("x-foo", class extends HTMLElement {
   of the ones specified in their description:</p>
 
   <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>
+   <li><p><span data-x="stack push">Push</span> a new <span>element queue</span> onto this object's
+   <span>relevant agent</span>'s <span>custom element reactions stack</span>.</p></li>
 
    <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>Let <var>queue</var> be the result of <span data-x="stack pop">popping</span> from this
+   object's <span>relevant agent</span>'s <span>custom element reactions stack</span>.</p></li>
 
    <li><p><span>Invoke custom element reactions</span> in <var>queue</var>.</p></li>
 
@@ -105883,8 +105890,8 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
      <li><p>If the <span>JavaScript execution context stack</span> is empty, then <span>perform a
      microtask checkpoint</span>.</p></li>
 
-     <li><p>Push a new <span>element queue</span> onto the <span>custom element reactions
-     stack</span>.</li>
+     <li><p>Push a new <span>element queue</span> onto <var>document</var>'s <span>relevant
+     agent</span>'s <span>custom element reactions stack</span>.</li>
     </ol>
    </li>
 
@@ -105919,9 +105926,9 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
     <p>If <var>will execute script</var> is true, then:</p>
 
     <ol>
-     <li><p>Let <var>queue</var> be the result of popping the <span>current element queue</span>
-     from the <span>custom element reactions stack</span>. (This will be the same <span>element
-     queue</span> as was pushed above.)</p></li>
+     <li><p>Let <var>queue</var> be the result of popping from <var>document</var>'s <span>relevant
+     agent</span>'s <span>custom element reactions stack</span>. (This will be the same
+     <span>element queue</span> as was pushed above.)</p></li>
 
      <li><p><span>Invoke custom element reactions</span> in <var>queue</var>.</p></li>
 
@@ -105981,15 +105988,15 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
 
     <ol>
      <li><p>If the parser was not created as part of the <span>HTML fragment parsing
-     algorithm</span>, then push a new <span>element queue</span> onto the <span>custom element
-     reactions stack</span>.</li>
+     algorithm</span>, then push a new <span>element queue</span> onto <var>element</var>'s
+     <span>relevant agent</span>'s <span>custom element reactions stack</span>.</li>
 
      <li><p>Insert <var>element</var> at the <var>adjusted insertion location</var>.</p></li>
 
      <li><p>If the parser was not created as part of the <span>HTML fragment parsing
-     algorithm</span>, then pop the <span>element queue</span> from the <span>custom element
-     reactions stack</span>, and <span>invoke custom element reactions</span> in that
-     queue.</p></li>
+     algorithm</span>, then pop the <span>element queue</span> from <var>element</var>'s
+     <span>relevant agent</span>'s <span>custom element reactions stack</span>, and <span>invoke
+     custom element reactions</span> in that queue.</p></li>
     </ol>
 
     <p class="note">If the <var>adjusted insertion location</var> cannot accept more