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

This first thing to note is that user agent sniffing does not just mean looking at navigator.userAgent, it is a general term to describe the large array of methods that people use to change behaviour based on what they believe the browser is.

So the problem is not looking at the user agent string, the problem is deciding what your site should do based on what you think the browser is. This means you may unavoidably limit or break your site in the future; For instance I have seen multiple canvas demos which block IE. They aren't checking to see whether canvas is supported, they are explicitly looking for IE, and if they see it they say IE is broken, this means that even if IE did eventually support canvas these sites still wouldn't work.

Rather than browser sniffing you should always attempt to detect the feature or bug you are interested in. The most common example of these tests is "object detection", eg. document.createElement("canvas").getContext is how the existence of canvas should be detected, and will correctly pick up canvas in any browser, even if current versions don't support it.