With KernelEx updates by Jumper on Windows 98SE, you can go up to 2.8 with SeaMonkey, theoretically you can try 2.9, but it will crash a lot more often and will not work well.
SeaMonkey 2.9 is the last version for vanilla Windows 2000, and SeaMonkey 1.1.19 the last for Windows 98/ME. Both useless for proper HTML. As a test, I try to load my main daily news website; If the blocks are rendered exactly like I see on my macOS, then it's working as expected. If they render in cascade (meaning CSS is not rendering properly or some HTML5 tags aren't being recognized) then it's garbage to me (unless the layout looks sort of a mobile website as a fallback, then it's acceptable).
With BlackWingCat's kernel extenders on Windows 2000, I can go to the latest (and I really mean, latest) SeaMonkey: 2.49.5, and they announced that it will be the last version supporting Windows XP and Vista. "How rude". I was becoming a fan of SeaMonkey after that until I learned this news.
At least Youtube works, sites are rendered as expected. Probably some super new CSS3 properties aren't, those that are in the draft for the next version of CSS. Hopefully - and I haven't tested yet - CSS Grid & Flexbox are still supported.
If KernelEx with updates on Windows 98 is supposed to make possible to run XP-like apps, then why it doesn't work in practice for SeaMonkey? Something must be missing.
The most decent browser I could run on Windows 98SE vanilla was probably Retrozilla 2.2. No Youtube, and no proper rendering of my favorite websites, though. And some cipher overlap errors somewhere else. Opera 10.63 probably was a little better. I haven't tried other browsers in a vanilla Windows 98 apart of RetroZilla, SeaMonkey and Opera. I know Firefox 2.x won't render OK.
