What needs to be supported in Safari to make it C3 work.

C3 is awesome. I cannot wait until Safari on mobile supports the appropriate code to make it work.

What specifically should I look out for in terms of what Safari should support. I think CSS grids was one item. Are there others?

Safari 11 already has the absolute minimum C3 needs to run, and we do have an experimental version of C3 running in it (see this tweet). There's still some work to do to polyfill missing features, work around Safari bugs, and make sure performance is good enough. However we can probably at least bring out a sort of alpha-test of C3 for Safari after the Safari 11 release.