Rename colspan back to columnspan.

See #198 (comment)
and issue #180

index.html

@@ -4581,11 +4581,11 @@ <h4>Entry in Table or Matrix <code>&lt;mtd&gt;</code></h4>
             in <a href="#global-attributes"></a> as well as the following attributes:
           </p>
           <ul id="mtd-attributes">
-            <li><a>colspan</a></li>
+            <li><a>columnspan</a></li>
             <li><a>rowspan</a></li>
           </ul>
           <p>
-            The <dfn><code>colspan</code></dfn> (respectively
+            The <dfn><code>columnspan</code></dfn> (respectively
             <dfn><code>rowspan</code></dfn>) attribute has the same
             syntax and semantics as the
             [^td/colspan^]
@@ -4594,12 +4594,13 @@ <h4>Entry in Table or Matrix <code>&lt;mtd&gt;</code></h4>
             attribute on the <code>&lt;td&gt;</code> element from [[HTML]].
             In particular, the parsing of these attributes is handled as
             described in the
-            <a data-cite="HTML/../#algorithm-for-processing-rows">algorithm for processing rows</a>.
+            <a data-cite="HTML/../#algorithm-for-processing-rows">algorithm for processing rows</a>, always reading "<code>colspan</code>" as
+            "<code>columnspan</code>".
           </p>
           <div class="note">
             The name of the column spanning attribute in [[MathML3]] and earlier
-            versions was <code>columnspan</code>. For consistency with [[HTML]],
-            this specification uses <code>colspan</code> instead.
+            versions is <code>columnspan</code> and is preserved for backward
+            compatibility reasons.
           </div>
           <p>
             The <code>&lt;mtd&gt;</code> element