I just write standards compliant ES6 code, put it through a transpiler and some polyfills, and my code runs on every browser. I haven't had to think about which browser I'm targeting for years.
... but you can (and I'd argue in a lot of cases, looking around at the overly-complicated sites on the web, should) write a lot of fully functional web pages by keeping to the standard and trusting it's good enough. We are no longer in the bad old days where IE and other browsers deviated significantly---most of the browsers today adhere closely to the spec, and in the places they don't, it's often more like one browser handles a nasty corner case better than another because the browser built an optimization in for a critical path your page layout cares about.