Rewrite output element to not use non-existing observers

The output element was defined to use descendant observers, which do not exist in all implementations and are not grounded in a specification concept. Thankfully, it can also be written to not require those.

Tests: web-platform-tests/wpt#16080.

Fixes #4163.

Author: annevk

source

@@ -3193,7 +3193,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x="concept-slot" data-x-href="https://dom.spec.whatwg.org/#concept-slot">slot</dfn> concept, and its <dfn data-x="slot-name" data-x-href="https://dom.spec.whatwg.org/#slot-name">name</dfn> and <dfn data-x-href="https://dom.spec.whatwg.org/#slot-assigned-nodes">assigned nodes</dfn></li>
      <li>The <dfn data-x="finding flattened slotables" data-x-href="https://dom.spec.whatwg.org/#find-flattened-slotables">find flattened slotables</dfn> algorithm</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#assign-a-slot">assign a slot</dfn> algorithm</li>
-     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-node-pre-insert">pre-insert</dfn>, <dfn data-x="concept-node-insert" data-x-href="https://dom.spec.whatwg.org/#concept-node-insert">insert</dfn>, <dfn data-x="concept-node-append" data-x-href="https://dom.spec.whatwg.org/#concept-node-append">append</dfn>, <dfn data-x="concept-node-replace" data-x-href="https://dom.spec.whatwg.org/#concept-node-replace">replace</dfn>, <dfn data-x="concept-node-replace-all" data-x-href="https://dom.spec.whatwg.org/#concept-node-replace-all">replace all</dfn>, <dfn data-x="concept-node-remove" data-x-href="https://dom.spec.whatwg.org/#concept-node-remove">remove</dfn>, and <dfn data-x="concept-node-adopt" data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt">adopt</dfn> algorithms for nodes</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-node-pre-insert">pre-insert</dfn>, <dfn data-x="concept-node-insert" data-x-href="https://dom.spec.whatwg.org/#concept-node-insert">insert</dfn>, <dfn data-x="concept-node-append" data-x-href="https://dom.spec.whatwg.org/#concept-node-append">append</dfn>, <dfn data-x="concept-node-replace" data-x-href="https://dom.spec.whatwg.org/#concept-node-replace">replace</dfn>, <dfn data-x="concept-node-replace-all" data-x-href="https://dom.spec.whatwg.org/#concept-node-replace-all">replace all</dfn>, <dfn data-x-href="https://dom.spec.whatwg.org/#string-replace-all">string replace all</dfn>, <dfn data-x="concept-node-remove" data-x-href="https://dom.spec.whatwg.org/#concept-node-remove">remove</dfn>, and <dfn data-x="concept-node-adopt" data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt">adopt</dfn> algorithms for nodes</li>
      <li>The <dfn data-x="concept-node-insert-ext" data-x-href="https://dom.spec.whatwg.org/#concept-node-insert-ext">insertion steps</dfn>,
              <dfn data-x="concept-node-remove-ext" data-x-href="https://dom.spec.whatwg.org/#concept-node-remove-ext">removing steps</dfn>,
              <dfn data-x="concept-node-adopt-ext" data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt-ext">adopting steps</dfn>, and
@@ -3203,6 +3203,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-element-attribute">attribute list</dfn> concept</li>
      <li>The <dfn data-x="concept-cd-data" data-x-href="https://dom.spec.whatwg.org/#concept-cd-data">data</dfn> of a text node</li>
      <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-child-text-content">child text content</dfn> of a node</li>
+     <li>The <dfn data-x-href="https://dom.spec.whatwg.org/#concept-descendant-text-content">descendant text content</dfn> of a node</li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#interface-event"><code>Event</code></dfn> interface</li>
      <li><dfn data-x-href="https://dom.spec.whatwg.org/#interface-eventtarget"><code>EventTarget</code></dfn> interface</li>
      <li>The <!--en-GB--><dfn id="activation-behaviour" data-x-href="https://dom.spec.whatwg.org/#eventtarget-activation-behavior">activation behavior</dfn> hook</li>
@@ -51653,89 +51654,85 @@ interface <dfn>HTMLOutputElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
-  <p>The element has a <dfn data-x="concept-output-mode">value mode flag</dfn> which is either <i
-  data-x="concept-output-mode-value">value</i> or <i
-  data-x="concept-output-mode-default">default</i>. Initially, the <span
-  data-x="concept-output-mode">value mode flag</span> must be set to <i
-  data-x="concept-output-mode-default">default</i>.</p>
+  <p>The element has a <dfn data-x="concept-output-default-value-override">default value
+  override</dfn> (null or a string). Initially it must be null.</p>
 
-  <p>The element also has a <dfn data-x="concept-output-defaultValue">default value</dfn>.
-  Initially, the <span data-x="concept-output-defaultValue">default value</span> must be the empty
-  string.</p>
+  <p>The element's <dfn data-x="concept-output-default-value">default value</dfn> is determined by
+  the following steps:</p>
 
-  <p>When the <span data-x="concept-output-mode">value mode flag</span> is in mode <dfn><i
-  data-x="concept-output-mode-default">default</i></dfn>, the contents of the element represent both
-  the value of the element and its <span data-x="concept-output-defaultValue">default value</span>.
-  When the <span data-x="concept-output-mode">value mode flag</span> is in mode <dfn><i
-  data-x="concept-output-mode-value">value</i></dfn>, the contents of the element represent the
-  value of the element only, and the <span data-x="concept-output-defaultValue">default value</span>
-  is only accessible using the <code data-x="dom-output-defaultValue">defaultValue</code> IDL
-  attribute.</p>
+  <ol>
+   <li><p>If this element's <span data-x="concept-output-default-value-override">default value
+   override</span> is non-null, then return it.</p></li>
 
-  <p>Whenever the element's descendants are changed in any way, if the <span
-  data-x="concept-output-mode">value mode flag</span> is in mode <i
-  data-x="concept-output-mode-default">default</i>, the element's <span
-  data-x="concept-output-defaultValue">default value</span> must be set to the value of the
-  element's <code>textContent</code> IDL attribute.</p>
+   <li><p>Return this element's <span>descendant text content</span>.</p></li>
+  </ol>
 
   <p>The <span data-x="concept-form-reset-control">reset algorithm</span> for <code>output</code>
-  elements is to set the element's <span data-x="concept-output-mode">value mode flag</span> to <i
-  data-x="concept-output-mode-default">default</i> and then to set the element's
-  <code>textContent</code> IDL attribute to the value of the element's <span
-  data-x="concept-output-defaultValue">default value</span> (thus replacing the element's child
-  nodes).</p>
+  elements is to run these steps:
+
+  <ol>
+   <li><p><span>String replace all</span> with this element's <span
+   data-x="concept-output-default-value">default value</span> within this element.</p></li>
+
+   <li><p>Set this element's <span data-x="concept-output-default-value-override">default value
+   override</span> to null.</p></li>
+  </ol>
 
   </div>
 
   <dl class="domintro">
-
    <dt><var>output</var> . <code subdfn data-x="dom-output-value">value</code> [ = <var>value</var> ]</dt>
-
    <dd>
-
     <p>Returns the element's current value.</p>
 
     <p>Can be set, to change the value.</p>
-
    </dd>
 
    <dt><var>output</var> . <code subdfn data-x="dom-output-defaultValue">defaultValue</code> [ = <var>value</var> ]</dt>
-
    <dd>
-
     <p>Returns the element's current default value.</p>
 
     <p>Can be set, to change the default value.</p>
-
    </dd>
 
    <dt><var>output</var> . <code subdfn data-x="dom-output-type">type</code></dt>
-
    <dd>
-
     <p>Returns the string "<code data-x="">output</code>".</p>
-
    </dd>
-
   </dl>
 
   <div w-nodev>
 
-  <p>The <dfn><code data-x="dom-output-value">value</code></dfn> IDL attribute must act like the
-  element's <code>textContent</code> IDL attribute, except that on setting, in addition, before the
-  child nodes are changed, the element's <span data-x="concept-output-mode">value mode flag</span>
-  must be set to <i data-x="concept-output-mode-value">value</i>.</p>
+  <p>The <dfn><code data-x="dom-output-value">value</code></dfn> attribute's getter must return this
+  element's <span>descendant text content</span>.</p>
+
+  <p>The <code data-x="dom-output-value">value</code> attribute's setter must run these steps:</p>
+
+  <ol>
+   <li><p>Set this element's <span data-x="concept-output-default-value-override">default value
+   override</span> to its <span data-x="concept-output-default-value">default value</span>.</p></li>
+
+   <li><p><span>String replace all</span> with the given value within this element.</p></li>
+  </ol>
+
+  <p>The <dfn><code data-x="dom-output-defaultValue">defaultValue</code></dfn> attribute's getter
+  must return the result of running this element's <span
+  data-x="concept-output-default-value">default value</span>.</p>
+
+  <p>The <code data-x="dom-output-defaultValue">defaultValue</code> attribute's setter must run
+  these steps:</p>
+
+  <ol>
+   <li><p>If this element's <span data-x="concept-output-default-value-override">default value
+   override</span> is null, then <span>string replace all</span> with the given value within this
+   element and return.</p></li>
 
-  <p>The <dfn><code data-x="dom-output-defaultValue">defaultValue</code></dfn> IDL attribute, on
-  getting, must return the element's <span data-x="concept-output-defaultValue">default
-  value</span>. On setting, the attribute must set the element's <span
-  data-x="concept-output-defaultValue">default value</span>, and, if the element's <span
-  data-x="concept-output-mode">value mode flag</span> is in the mode <i
-  data-x="concept-output-mode-default">default</i>, set the element's <code>textContent</code> IDL
-  attribute as well.</p>
+   <li><p>Set this element's <span data-x="concept-output-default-value-override">default value
+   override</span> to the given value.</p></li>
+  </ol>
 
-  <p>The <dfn><code data-x="dom-output-type">type</code></dfn> attribute must return the string
-  "<code data-x="">output</code>".</p>
+  <p>The <dfn><code data-x="dom-output-type">type</code></dfn> attribute's getter must return "<code
+  data-x="">output</code>".</p>
 
   <p>The <dfn><code data-x="dom-output-htmlFor">htmlFor</code></dfn> IDL attribute must
   <span>reflect</span> the <code data-x="attr-output-for">for</code> content attribute.</p>