Нашёл в справке по Qt:

QDomNode QDomNode::nextSibling () const
Returns the next sibling in the document tree. Changing the returned node will also change the node in the document tree.
If you have XML like this:
<h1>Heading</h1>
<p>The text...</p>
<h2>Next heading</h2>
and this QDomNode represents the <p> tag, nextSibling() will return the node representing the <h2> tag.

но про стандарт всё равно интересно, товарищи знатоки!