Editorial: abstract getting the common ancestor

Fixes #56.

Author: annevk

dom.bs

@@ -9217,19 +9217,24 @@ to set <a>this</a>'s <a for=range>start</a> and <a for=range>end</a> to
 </dl>
 
 <div algorithm>
-<p>The <dfn attribute for=Range><code>commonAncestorContainer</code></dfn> getter steps are:
+<p>To <dfn>get the common ancestor</dfn> of a <a>live range</a> <var>range</var>:
 
 <ol>
- <li><p>Let <var>container</var> be <a for=range>start node</a>.
+ <li><p>Let <var>container</var> be <var>range</var>'s <a for=range>start node</a>.
 
  <li><p>While <var>container</var> is not an <a for=tree>inclusive ancestor</a> of
- <a for=range>end node</a>: set <var>container</var> to <var>container</var>'s
+ <var>range</var>'s <a for=range>end node</a>: set <var>container</var> to <var>container</var>'s
  <a for=tree>parent</a>.
 
  <li><p>Return <var>container</var>.
 </ol>
 </div>
 
+<div algorithm>
+<p>The <dfn attribute for=Range><code>commonAncestorContainer</code></dfn> getter steps are to
+return the result of <a>getting the common ancestor</a> of <a>this</a>.
+</div>
+
 <hr>
 
 <div algorithm>
@@ -9587,10 +9592,8 @@ method steps are:
    <li>Return <var>fragment</var>.
   </ol>
 
- <li><p>Let <var>commonAncestor</var> be <var>originalStartNode</var>.
-
- <li><p>While <var>commonAncestor</var> is not an <a for=tree>inclusive ancestor</a> of
- <var>originalEndNode</var>: set <var>commonAncestor</var> to its own <a for=tree>parent</a>.
+ <li><p>Let <var>commonAncestor</var> be the result of <a>getting the common ancestor</a> of
+ <var>range</var>.
 
  <li><p>Let <var>firstPartiallyContainedChild</var> be null.
 
@@ -9792,10 +9795,8 @@ of a <a>live range</a> <var>range</var>:
    <li><p>Return <var>fragment</var>.
   </ol>
 
- <li><p>Let <var>commonAncestor</var> be <var>originalStartNode</var>.
-
- <li><p>While <var>commonAncestor</var> is not an <a for=tree>inclusive ancestor</a> of
- <var>originalEndNode</var>: set <var>commonAncestor</var> to its own <a for=tree>parent</a>.
+ <li><p>Let <var>commonAncestor</var> be the result of <a>getting the common ancestor</a> of
+ <var>range</var>.
 
  <li><p>Let <var>firstPartiallyContainedChild</var> be null.