This is a lot nicer than first_child_with_tag_name(...).
The is<T>(Node) functions are obviously unoptimized at the moment,
and this is about establishing pleasant patterns right now. :^)
You can now query Document::title() to get a String containing whatever
is inside the document's <title> tag.
In support of this, this patch adds the <html>, <head> and <title>
elements.