Anything starting '-' won't show up on Google, it will literally give you empty results, so that's why -webkit-match-parent gives nothing and webkit-match-parent works.
"-webkit-match-parent"
I suspect it trips up people that don't know this more than it's actually used.
Google should at least show a user friendly help message at the top of the page, that actually teaches users about the filtering feature, and lets you turn it off, like:
Results containing "webkit-match-parent" omitted because of "-" prefix. (Click to include.)
But that would reduce the ad surface area by a tiny amount, so oh well...