Valid user agent?
There's no such thing as a valid user agent, they're not regulated by anyone and neither should they be. Don't get me started on the stupidity of all major browsers still being labelled as Mozilla/5.0.
Why we can't just cut all this out and just use Firefox/56 or Chrome/127 or whatever we're up to now is beyond me.
In fact, I'm getting a user agent plugin now and updating mine to something sensible. I'm starting a revolution, even if it's just a single grumpy middle aged man revolution.