Update textarea textContent/defaultValue/value interaction

This fixes #2752, where it was revealed that most browsers do not care
about the textContent of a textarea, but instead about its child text
content. That is, if you use the DOM APIs to insert a child element into
the textarea element, its text does not show up for editing or as part
of the value and defaultValue properties (even though it shows up in
textContent).

This moves to a model where textareas operate entirely on the child text
content. This matches Blink and Gecko, and somewhat matches WebKit.

This also fixes #2750, by using the newly-introduced "child text content
change steps" hook introduced to DOM in
whatwg/dom#466.

source

@@ -1971,10 +1971,6 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
   or string, means that the length of the text is zero (i.e., not even containing <span
   data-x="control">controls</span> or U+0020 SPACE).</p>
 
-  <p>An element's <dfn data-export="">child text content</dfn> is the concatenation of the <span
-  data-x="concept-cd-data">data</span> of all the <code>Text</code> nodes that are children of the
-  element (ignoring any other nodes such as comments or elements), in <span>tree order</span>.</p>
-
   <p>A <dfn data-x="nodes are inserted">node <var>A</var> is inserted</dfn> into a node <var>B</var>
   when the <span data-x="concept-node-insert-ext">insertion steps</span> are invoked with
   <var>A</var> as the argument and <var>A</var>'s new parent is <var>B</var>. Similarly, a <dfn
@@ -3029,10 +3025,12 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <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="concept-element-attributes-change" data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-change">change</dfn>, <dfn data-x="concept-element-attributes-append" data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-append">append</dfn>, <dfn data-x="concept-element-attributes-remove" data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-remove">remove</dfn>, <dfn data-x="concept-element-attributes-replace" data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-replace">replace</dfn>, and <dfn data-x="concept-element-attributes-set-value" data-x-href="https://dom.spec.whatwg.org/#concept-element-attributes-set-value">set value</dfn> algorithms for attributes</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>, and
-             <dfn data-x="concept-node-adopt-ext" data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt-ext">adopting steps</dfn> hooks</li>
+             <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
+             <dfn data-x-href="https://dom.spec.whatwg.org/#concept-node-adopt-ext">child text content change steps</dfn> hooks</li>
      <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><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>
@@ -50481,15 +50479,15 @@ interface <dfn>HTMLTextAreaElement</dfn> : <span>HTMLElement</span> {
   elements must propagate the <span data-x="concept-textarea-raw-value">raw value</span> and <span
   data-x="concept-fe-dirty">dirty value flag</span> from the node being cloned to the copy.</p>
 
-  <p>When the <code>textarea</code> element's <code>textContent</code> IDL attribute changes value,
-  if the element's <span data-x="concept-fe-dirty">dirty value flag</span> is false, then the
-  element's <span data-x="concept-textarea-raw-value">raw value</span> must be set to the value of
-  the element's <code>textContent</code> IDL attribute.</p>
+  <p>The <span>child text content change steps</span> for <code>textarea</code> elements must, if
+  the element's <span data-x="concept-fe-dirty">dirty value flag</span> is false, set the element's
+  <span data-x="concept-textarea-raw-value">raw value</span> to its <span>child text
+  content</span>.</p>
 
   <p>The <span data-x="concept-form-reset-control">reset algorithm</span> for <code>textarea</code>
   elements is to set the <span data-x="concept-fe-dirty">dirty value flag</span> back to false, and
-  set the <span data-x="concept-textarea-raw-value">raw value</span> of element to the value of the
-  element's <code>textContent</code> IDL attribute.</p>
+  set the <span data-x="concept-textarea-raw-value">raw value</span> of element to its <span>child
+  text content</span>.</p>
 
   <p>When a <code>textarea</code> element is popped off the <span>stack of open elements</span> of
   an <span>HTML parser</span> or <span>XML parser</span>, then the user agent must invoke the
@@ -50750,8 +50748,9 @@ interface <dfn>HTMLTextAreaElement</dfn> : <span>HTMLElement</span> {
   <p>The <dfn><code data-x="dom-textarea-type">type</code></dfn> IDL attribute must return the value
   "<code data-x="">textarea</code>".</p>
 
-  <p>The <dfn><code data-x="dom-textarea-defaultValue">defaultValue</code></dfn> IDL attribute must
-  act like the element's <code>textContent</code> IDL attribute.</p>
+  <p>The <dfn><code data-x="dom-textarea-defaultValue">defaultValue</code></dfn> IDL attribute must,
+  on getting, return the element's <span>child text content</span>. On setting, it must act as the
+  setter for the element's <code>textContent</code> IDL attribute.</p>
 
   <p>The <dfn><code data-x="dom-textarea-value">value</code></dfn> IDL attribute must, on getting,
   return the element's <span data-x="concept-fe-api-value">API value</span>. On setting, it must