Element containers for OPTION elements are hard.

Let's go shopping. Or, alternatively, try and find the first
ancestor datalist element. If that doesn't exist, then use
the first ancestor select element.

Closes #735

Author: shs96c

webdriver-spec.html

@@ -4717,17 +4717,29 @@ <h2>Element State</h2>
  <li><p>Return false.
 </ol>
 
-<p>An <a>element</a>’s <dfn>container</dfn> is:
+<p>An <a><var>element</var></a>’s <dfn>container</dfn> is:
 
 <dl class=switch>
  <dt><a><code>option</code> element</a> in a valid <a>element context</a>
- <dd><a><code>optgroup</code> element</a> in a valid <a>element context</a>
- <dd><p>The <a>element</a>’s <a>element context</a>,
-  meaning its immediate parent node
-  in reverse tree-order originating from <a>element</a>
-  that is either a <a><code>datalist</code> element</a>
-  or a <a><code>select</code> element</a>.
-
+ <dt><a><code>optgroup</code> element</a> in a valid <a>element context</a>
+ <dd><p>The <a><var>element</var></a>'s <a>element context</a>, which
+  is determined by:
+  <ol>
+   <li><p>Let <var>datalist parent</var> be the
+    first <a><code>datalist</code> element</a> reached by traversing
+    the tree in reverse order from <var>element</var>,
+    or <a><code>undefined</code></a> if the root of the tree is
+    reached.
+   <li><p>Let <var>select parent</var> be the
+    first <a><code>select</code> element</a> reached by traversing
+    the tree in reverse order from <var>element</var>,
+    or <a><code>undefined</code></a> if the root of the tree is
+    reached.
+   <li><p>If <var>datalist parent</var>
+    is <a><code>undefined</code></a>, the <a>element context</a>
+    is <var>select parent</var>. Otherwise, the <a>element context</a>
+    is <var>datalist parent</var>.
+  </ol>
  <dt><a><code>option</code> element</a> in an invalid <a>element context</a>
  <dd><p>The element does not have a container.