On a panel of 4 Javascript library developers at Ajax Experience 2008, a question came up about how their libraries use browser detection. When John Resig suggested that libraries should strive for full feature detection (hardly used at all at the time) instead of browser/object detection, the other developers reacted like he was crazy. They mentioned the cases where this just isn’t possible, but none of the developers mentioned the huge, very good reason to do this whenever possible: There are pages using libraries deployed everyday that never get maintained. Yes, when a new browser changes behavior the libraries can quickly update their codebase, but many pages will never get those updates.

Note, there are autoloaders that may maintain the latest library version, but this doesn’t guarantee a stable library API over time. Feature detection (mostly) does.