You should be able to set ENABLE_CSS3_CONDITIONAL_RULES and build. Currently, there are build errors.
Created attachment 233937 [details]
Simplest patch to get the feature building on Mac. Still learning about the code, so there may be some additional fixup necessary.
Comment on attachment 233937 [details]
Clearing flags on attachment: 233937
Committed r170539: <http://trac.webkit.org/changeset/170539>
All reviewed patches have been landed. Closing bug.