HTML5/CSS3/ES6 define how to process legacy content as well as current — the only actual branching for versions is quirks mode and that has relatively minor eff

HTML5/CSS3/ES6 define how to process legacy content as well as current — the only actual branching for versions is quirks mode and that has relatively minor effects which aren't that much code to implement. Edge should, in principle, support everything, including legacy sites (at least as well as other browsers do today). Trident is only being kept around for sites that do extensive UA sniffing and rely on obscure behaviour — and pretty much only for the intranet.