Currently, we add prefixes to CSS properties when importing tests from WPT. This rewriting of the tests is fragile and the cause of a number of problems, and we've long wanted to get rid of this. At this point, we have ~200 tests where our prefixing is important, and we're unlikely to be moving these prefixed properties to be closer to the spec or fixing many bugs when new tests are added (though we may well do this when adding support for the unprefixed properties). As such, we may as well just fork the tests into http/wpt and remove the prefixing from the import script, which also allows us to remove the parse/serialize cycle we do which breaks a number of tests (bug 268218, bug 207731, bug 207736).