fix validation error

<li> elements can not be wrapped by <del> (or <ins>)
so I had to keep the element in place, with some placeholder text explaining that this step is gone.
The idea is to remove it entirely when the candidate correction is accepted.

Author: pchampin

index.html

@@ -2123,8 +2123,10 @@ <h3>Algorithm</h3>
                   being processed.</li>
               </ol>
             </li>
-            <del cite="#change_pr639">
-            <li class="changed">Otherwise, if <var>active context</var> has a
+            <li class="changed">
+              <ins cite="#change_pr639"><em>(this step was removed by a <a href="#change_pr639">candidate correction</a>)</em></ins>
+              <del cite="#change_pr639">
+              Otherwise, if <var>active context</var> has a
               <a>default base direction</a>:
               <ol>
                 <li>Initialize a variable <var>lang dir</var>
@@ -2141,8 +2143,8 @@ <h3>Algorithm</h3>
                   create one and set its value to the <a>term</a>
                   being processed.</li>
               </ol>
+              </del>
             </li>
-            </del>
             <li>Otherwise:
               <ol>
                 <li>If <var>language map</var> does not have a <var>default language</var> <a>entry</a>,<del cite="#change_pr639">