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Caption
Web dev: What browser is visiting the page?
User agent string:
A screenshot of a browser. The URL bar reads firefox://settings, a button on the URL bar is labelled Netscape, a popup from the button reads: "You're viewing a secure Opera page", and the web page title reads "Chrome settings".
Functionally useless. With the web standardized, we shouldn't need user agents anyway. It would be more beneficial to ask "do you support X, Y, and Z?"
It's called feature detection and it goes a long way back, even before Modernizr popularized it.
Popularized? That gets less than 100k downloads a week
It used to be huge.