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

Because if you get it wrong, you may accidentally cut off features to future browsers that support them.

I often find it useful. I know IE6 doesn't support alpha transparency, so I use browser sniffing to detect IE6 and hide/change elements that use them.

Also for running the mouse over quickly numerous times, try HoverIntent. It users setTimeout() I believe to only fire events when the mouse has been over an element for a brief period of time, saving cycles and avoiding queuing up of events and potentially freezing the browser.

Personally, I preferred jQuery with the browser version/type methods. It could be used to show a friendly hello message based on browser. Maybe jQuery deprecated it due to pressure that 'browser sniffing is evil'.

Update

This is what John Resig (creator of jQuery) says:

We're keeping jQuery.browser for the foreseeable future but we want developers to move away from using it - and the best way to get developers to do that is to become a good example of proper development patterns.

To be clear: The points included in $.support are mostly browser bug specific (IE bugs that can't be tested with normal object detection) - and they don't encompass every possible bug (only about a dozen or so). It's expected that other developers will add their own testing points in the future.

Also, in that commit I forgot to land the actual support.js file - it can be found here: http://dev.jquery.com/browser/trunk/jquery/src/support.js?rev=5986

Source: http://www.reddit.com/r/programming/comments/7l2mr/jquery_removes_all_browser_sniffing/

Also see: http://dev.jquery.com/changeset/5985