Differentiate between "children change" steps and "children inserted" steps.

See whatwg/html#12279

Author: noamr

dom.bs

@@ -2840,6 +2840,12 @@ that all pending <a>node tree</a> insertions completely finish before more inser
 <a for=/>remove</a>, and <a for=/>replace data</a>.
 </div>
 
+<div algorithm>
+<p><a lt="other applicable specifications">Specifications</a> may define
+<dfn export id=concept-node-children-inserted-ext>children inserted steps</dfn> for all or some
+<a for=/>nodes</a>. The algorithm is passed no argument and is called from <a for=/>insert</a.
+</div>
+
 <div algorithm=insert>
 <p>To <dfn export id=concept-node-insert>insert</dfn> a <a for=/>node</a> <var>node</var> into a
 <a for=/>node</a> <var>parent</var> before null or a <a for=/>node</a> <var>child</var>, with an
@@ -2952,7 +2958,8 @@ optional boolean <dfn for="insert"><var>suppressObservers</var></dfn> (default f
  <li><p>If <var>suppressObservers</var> is false, then <a>queue a tree mutation record</a> for
  <var>parent</var> with <var>nodes</var>, « », <var>previousSibling</var>, and <var>child</var>.
 
- <li><p>Run the <a>children changed steps</a> for <var>parent</var>.
+ <li><p>Run the <a>children inserted steps</a> for <var>parent</var>. If those are not defined, run
+ the <a>children changed steps</a> for <var>parent</var>.
 
  <li>
   <p>Let <var>staticNodeList</var> be a <a for=/>list</a> of <a for=/>nodes</a>, initially « ».</p>