Re: re: Not a defence, it's not difficult to make things work cross browser.
"It's obvious that you weren't working in...."
Wait what? I think present times are implied here and even when jquery was still a thing to use, you still had non-standard... standard methods that even old jquery wound up including. TODAY, I can't help to agree that if you need to sniff the agent, you've lost. But do you feel like a winner (that's the trick)?
Spoofing your users agent is a sign of somebody breaking things somewhere.
That said, today browser extensions are the new "Quirks Mode". There's more fucked up shit going on with "frameworks" and their interactions with browsers than people care to talk about, and every single one of those are proprietary regardless of origin.
But to solve all this, everyone could stop willing giving their users identity to Google, Facebook, Amazon, etc. and do proper design.