Nowadays the biggest issues are with CSS, and those frameworks won't help with that. However, there are other CSS frameworks (Bootstrap/Tailwind/etc) that do.

Nowadays the biggest issues are with CSS, and those frameworks won't help with that. However, there are other CSS frameworks (Bootstrap/Tailwind/etc) that do. Even using those, I'd still give other browsers a look over to see if anything looks odd. One weird one we had was, on Chrome/Edge a React dropdown looked fine, but on Firefox it was somehow positioned like 80px too high. I don't remember exactly what the fix was, just that it was a CSS problem and the original developer was relying on a Chrome default they didn't realize was there and not in other browsers. Something akin to "position: relative; top: 20px;" just happened to look vertically centered, but only in Chrome-based browsers.