In this case, it looks like he's optimizing for Firefox and Chrome--he mentions benchmark results from both and also running into a SpiderMonkey optimization th

In this case, it looks like he's optimizing for Firefox and Chrome--he mentions benchmark results from both and also running into a SpiderMonkey optimization that was causing problems. That does leave out Safari, Edge, Opera, and probably some other obscure browsers, and doesn't help with your point about future engine changes potentially breaking the optimizations.