Can I use... is the standard tool for assessing browser support. It shows (among many other things) what fraction of users have browsers supporting a given feature, and allows importing a Google Analytics or Simple Analytics export file of site visits so you can use your own website's browser distribution instead of whatever global default they choose.
It would be great if such files were available for Wikimedia site users (even better if there were separate readers / editors files as a lot of our code is targeted at editors only and it's a fairly different demographic), so we could easily check how many of our users are cut off from using a given feature.