We went through this 15 years ago with custom scroll bars that didn't look like scroll bars, didn't operate like scroll bars and didn't interact like scroll bars, all because it could happen. An Ian Malcom quote comes to mind.
- I can't get this exact shade of cornflower blue
- Oh, here's a solution that works! Oh, it doesn't work in all browsers
- Hmm, here's a stack overflow answer that says use a div. It's 5 years old, is that current? Dunno but it has 457 up votes and works
Some people are concerned about accessibility but the JavaScript doesn't work in iOS. Others have iOS support but they screwed up cache headers and it's super slow on any low bandwidth connection.
Balancing all those things (and that's a very incomplete list) is hard and it's not fair to expect side/hobby/oss projects to do it 100%.
Being really good at front-end (to the point you know when to use a native control or not, which "standard" features you can rely on, how to think about accessibility+responsive+dark mode+ad blockers+low bandwidth+seo etc) is an entire discipline that takes a lot of effort to stay current in.
If you use pure, standard html with default styling you still end up with a page using bad colors and Times New Roman that looks straight out of the 90s. If that's not the look you're going for, you need to do some customization (via CSS) and the difference between "some" and "to the nines" is not exactly a clear line -- even to people that have specialized their careers in front-end dev.