I use Firefox. Safari is used exclusively on IOS and pretty dominant on MacOS. HTML5 works fine across those. The web app we develop has occasional issues with Safari that we need to look into. Things not working in both Firefox and Chrome is extremely rare for us and usually fixed pretty easily in the rare case we notice it.

I remember having to deal with xhtml, html 4, Netscape, Opera, Internet Explorer, etc. a world of pain.

Even by 2009, things were already massively improved (with the exception of IE of course). WhatWg was founded in 2004 and that's when work started on HTML5. That's before Chrome was a thing. That didn't happen until 2008 and would have not been possible without decent specifications provided by WhatWg. Apple and Mozilla were working together on that one.

Chrome happened because of HTML 5; not the other way around.

> The web app we develop has occasional issues with Safari that we need to look into. Things not working in both Firefox and Chrome is extremely rare for us and usually fixed pretty easily in the rare case we notice it.

FWIW, that mirrors my experience entirely. i primarily develop with FF and test with Chrome/Chromium as a sanity check. Safari, once the app is out the wild, is always the outlier (and i don't use it, so can't fix its shortcomings except via trial and error).