Comment by cosmic_cheese

> But browsers (and browser technologies) have documented track of being fully backward compatible up to the beginnings of WWW, and it's not going to change.

That can however be undermined if web apps are poorly built and depend on quirks and behaviors specific to a particular engine (or in some cases, even particular versions of a particular engine) in order to function.

So I would say this benefit applies specifically to web apps that thoroughly apply KISS — that is, using only the most boring, solidified, widely supported APIs and favoring robustness over bells and whistles — and make a point of testing against all three major engines. Those apps will likely stand the test of time and run even under future new engines. On the other hand, the ones with severe shiny API syndrome that only ever get tested against the latest Chrome are probably much more brittle and more likely to be broken N years after abandonment.