> And detecting the browser for obsolete browsers is usually a perfectly fine solution.

But there are a long tail of user agents and the average web developer does a terrible job at identifying those in the long tail. My password manager on iOS uses the newest Safari engine, but all websites think it is an outdated/obsolete browser because it uses a user agent string they don't recognize.

Also, my bank doesn’t use highly interactive features you mention. It’s almost exclusively links, forms and a little validation JavaScript. Feature sniffing would have been a much better experience.