> JS and Python wouldn't be what they are today if you had to `./configure` every website you want to visit, lmao.

You just gave me a flashback to the IE6 days. Yes, that's precisely what we did. On every page load.

It's called "feature detection", and was the recommended way of doing things (the bad alternative was user agent sniffing, in which you read the user agent string to guess the browser, and then assumed that browser X always had feature Y; the worst alternative was to simply require browser X).