Clarify label.form IDL attribute

Fixes #1308.

source

@@ -41314,9 +41314,19 @@ interface <dfn>HTMLFormElement</dfn> : <span>HTMLElement</span> {
   <p>The <dfn><code data-x="dom-label-control">control</code></dfn> IDL attribute must return the
   <code>label</code> element's <span>labeled control</span>, if any, or null if there isn't one.</p>
 
-  <p>The <dfn><code data-x="dom-label-form">form</code></dfn> IDL attribute must return the
-  <code>label</code> element's <span>labeled control</span>'s <span>form owner</span>, or null if
-  there isn't a <span>labeled control</span> or there is but is has no <span>form owner</span>.</p>
+  <p>The <dfn><code data-x="dom-label-form">form</code></dfn> IDL attribute must run the following
+  steps:</p>
+
+  <ol>
+   <li><p>If the <code>label</code> element has no <span>labeled control</span>, then return
+   null.</p></li>
+
+   <li><p>If the <code>label</code> element's <span>labeled control</span> is not a
+   <span>form-associated element</span>, then return null.</p></li>
+
+   <li><p>Return the <code>label</code> element's <span>labeled control</span>'s <span>form
+   owner</span> (which can still be null).</p></li>
+  </ol>
 
   </div>