[cssom] Please review CSSOM 4. Selectors

A few months ago the CSSWG agreed to review a section each month of CSSOM.  
Let's start now!

Please review this section:

     https://drafts.csswg.org/cssom/#selectors

It's short and not full of known issues. :-)

Tests:

https://github.com/w3c/csswg-test/blob/master/cssom-1/selectorSerialize.html
https://github.com/w3c/csswg-test/blob/master/cssom-1/serialize-namespaced-type-selectors.html

Bugs:

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24115 (recently closed)

cheers,
-- 
Simon Pieters
Opera Software