The querySelector() methods on the Document, DocumentFragment, and Element interfaces must
return the first matching Element node within the subtrees of the context
node. If there is no matching Element, the method must return null.
The querySelectorAll() methods on
the Document, DocumentFragment, and Element interfaces must
return a NodeList containing all of
the matching Element nodes within the
subtrees of the context
node, in document order. If there are no
matching nodes, the method must return an empty NodeList.