Fix live range updating logic

dom.bs

@@ -2910,16 +2910,17 @@ a <var>child</var>, run these steps:
 
  <li><p>Let <var>index</var> be <var>node</var>'s <a for=tree>index</a>.
 
- <li><p>For each <a>live range</a> whose <a for=range>start node</a> is an
- <a>inclusive descendant</a> of <var>node</var>, set its <a for=range>start</a> to
- (<var>parent</var>, <var>index</var>).
-
- <li><p>For each <a>live range</a> whose <a for=range>end node</a> is an <a>inclusive descendant</a>
- of <var>node</var>, set its <a for=range>end</a> to (<var>parent</var>, <var>index</var>).
-
- <li><p>For each <a>live range</a> whose <a for=range>start node</a> is <var>parent</var> and
- <a for=range>start offset</a> is greater than <var>index</var>, decrease its
- <a for=range>start offset</a> by 1.
+ <li>
+  <p>For each <a>live range</a> whose <a for=range>start node</a> is <var>parent</var> and
+  <a for=range>start offset</a> is greater than <var>index</var>, decrease its
+  <a for=range>start offset</a> by 1.</p>
+
+  <p class="note">Note that unlike the traditional <a for=/ lt="remove">removal</a> case, we do not
+  need to update <a>live range</a> state when their <a for=range>start node</a> or
+  <a for=range>end node</a> is an <a>inclusive descendant</a> of the <var>node</var>. This is
+  because said <a>nodes</a> do not get removed from their <a>tree</a>, so ranges associated with
+  them stay intact.</p>
+ </li>
 
  <li><p>For each <a>live range</a> whose <a for=range>end node</a> is <var>parent</var> and
  <a for=range>end offset</a> is greater than <var>index</var>, decrease its
@@ -3004,13 +3005,6 @@ a <var>child</var>, run these steps:
 
  <li><a>Queue a tree mutation record</a> for <var>parent</var> with <var>nodes</var>, « »,
  <var>previousSibling</var>, and <var>child</var>.</p></li>
-
- <li>
-  <p>Run the <a>children changed steps</a> for <var>parent</var>.</p>
-
-  <p class="XXX">TODO(domfarolino): Before merging, do a full audit on children changed steps, and
-  write tests.</p>
- </li>
 </ol>