Browser incompatibilities are mostly gone, where they still exist frameworks tend to do nothing to solve them. I write web stuff and I used to do the dance of testing with a bunch of browsers and fixing stuff. Since IE 11 there have been basically nothing to fix, the browsers really do behave the same.
Not really, and not even something that pertains to "really old" browsers.
As a somewhat recent example, in order to implement "SameSite=None" properly, one has to ensure it's _not_ sent to browsers (including recent Safaris and Chromes) which implement it badly. See also: https://www.chromium.org/updates/same-site/incompatible-clie...
