Then, using your analytics package (which you have, correct?) You run testing either on actual hardware, or something like Browserstack for everything else in descending order of popularity by unique visitor.

Really, you have several different rendering engines and JS engines in play at any given time - and it's helpful to have an agreed upon support model in place prior to starting development. Generally speaking, support anything with 5+% in the past 90, and make a argument for not spending time supporting things below that without very good cause.