[ACTION-413] CSS and XPath selectors conversion libraries

Hi,

quick search revealed several existing libraries which can translate CSS
selectors to XPath:

http://css2xpath.appspot.com/

https://bitbucket.org/MostThingsWeb/css2xpath/wiki/Home

http://nokogiri.org/Nokogiri/CSS.html

http://symfony.com/doc/2.0/components/css_selector.html

So, even without native CSS implementations we can keep CSS selectors in
the spec and say that it's convenient for authors and on implementation
side this can be done using existing XPath based implementations which
will simply convert CSS selectors to XPath expressions.

    Jirka

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