IIRC, Angular already ships with its own similar polyfill, so I'd guess it tries to fallback to native if possible. React doesn't really concern itself with styles anyway, though I guess they might need/want to make sure they're not standing in the way of using the shadow DOM.

Of the less-major ones, I only know that Polymer uses it - which makes sense, since this is one of the APIs it was explicitly created to push.