spoofing the user agent works fine

I'm continually upset by this. If you know you rely on, for example, the fetch() api, test for the fetch api[0]. Whitelisting browser user agents just means chasing your tail for forever.

By all means, politely warn a user that "your browser is not tested". It's getting to feel like a marketing driven decision, where pages just about say "our site is so powerful, we only work with the greatest browser ever, Chrome, so come back when you have it".

A local airbnb competitor currently does this.

[0] https://philipwalton.com/articles/loading-polyfills-only-whe...