The web version of html5 tanki online does not work for my mac and instead is stuck on this display: Any ideas what the issue is and how I can fix it?

If I download Tanki Online from steam, Can I access special missions or do I have to be on html5 to claim special missions? I would think yes because it wouldn't make sense to force people to use html5 but rather for them to stop using flash but im not sure.