Why is browser sniffing not a recommended practice?

Question

You hear it all over the place: using javascript to sniff the user agent string to detect browser versions is a Very Bad Thing. The latest version of jQuery has now deprecat

Answer 1

It's not recommended because browsers lie about who they are. See http://farukat.es/journal/2011/02/499-lest-we-forget-or-how-i-learned-whats-so-bad-about-browser-sniffing (an article by the author of the modernizr library).

Also, it is inherently not future-proof. From the article:

Browser sniffing is a technique in which you make assumptions about how a piece of code will work in the future. Generally this means making an assumption that a specific browser bug will always be there - which frequently leads to code breaking when browsers make changes and fix bugs.

Feature detection is recommended as it's much simpler, makes for clearer intent in code, and avoids the issue entirely of browsers lying about who they are.