Add and export get an element by ID

This is useful for HTML.

Author: annevk

dom.bs

@@ -3386,12 +3386,18 @@ DocumentFragment includes NonElementParentNode;
 </dl>
 
 <div algorithm>
-<p>The <dfn method for=NonElementParentNode><code>getElementById(<var>elementId</var>)</code></dfn>
-method steps are to return the first <a for=/>element</a>, in <a>tree order</a>, within
-<a>this</a>'s <a for=tree>descendants</a>, whose <a for=Element>ID</a> is <var>elementId</var>;
+<p>To <dfn export>get an element by ID</dfn>, given a <a for=/>node</a> <var>node</var> and a
+string <var>elementId</var>, return the first <a for=/>element</a>, in <a>tree order</a>, within
+<var>node</var>'s <a for=tree>descendants</a>, whose <a for=Element>ID</a> is <var>elementId</var>;
 otherwise, if there is no such <a for=/>element</a>, null.
 </div>
 
+<div algorithm>
+<p>The <dfn method for=NonElementParentNode><code>getElementById(<var>elementId</var>)</code></dfn>
+method steps are to return the result of <a>getting an element by ID</a> given <a>this</a> and
+<var>elementId</var>.
+</div>
+
 
 <h4 id=mixin-documentorshadowroot>Mixin {{DocumentOrShadowRoot}}</h4>