Elements array and query/queryAll methods

Re:
http://www.w3.org/TR/domcore/#elements
http://www.w3.org/TR/domcore/#parentnode

I'm looking any resources that elaborate on the Elements collection and the
query and queryAll methods of the ParentNode interface.

My understanding:

   - Elements is a subclass of Array, so we have all the mutator functions
   the HTMLCollection and NodeList lack.
   - query and queryAll allow CSS combinators to be used with the node they
   are invoked on.
   - query and queryAll return Elements collection.

Questions:

   - What use cases is Elements intended to solve the NodeList and
   HTMLCollection aren't suited for?
   - What other functions return an Elements object?
   - Will there be a "live" Elements array?
   - Is Elements intended to replace the use of NodeList and HTMLCollection?
   - Where can I find the UA implementation status for these features?
   - Does anyone know of a polyfill for either?

Thanks in advance for anyone who is able to point me in the right
direction. I haven't been able to find anything other than the specs I
linked to above.

- Eric Devine