I can't play using Safari 7.0.1 on OS X 10.9.1. (And by "can't play", I mean I see the "CodeCombat requires a modern browser to play" screen.)

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_1) AppleWebKit/537.73.11 (KHTML, like Gecko) Version/7.0.1 Safari/537.73.11