Polyfills can be quite expensive on a client device depending on their complexity, so they’re not some kind of free lunch.

Expensive in a few ways. Deferring styles to execution time means users on slow networks or devices will have an absolutely brutal load experience.

This might all be an acceptable trade off depending on one's user base, of course!

I don’t think there is a generally accepted implemented one.

They’d also be pretty expensive on the client