Which browsers should we support and test for?

Looks like React itself is going through this same issue these days: https://github.com/facebook/react/issues/9301

So far they've only said IE9+, but the issue above is suggesting a more concrete list of browsers would be better.

Current list goes something like:

Definitely supported with full support:
Chrome 49 (last available on XP/Vista)
Firefox ESR
IE 11
Safari 10
Edge 14

Probably supported with full support:
Safari 7
Chrome 41 (seeing a bunch of traffic here on Windows 7, not sure why)

Probably OK not to support:
Safari 6
IE9 (not 100% sure on this)