Nonetheless, its still mildly concerning that smaller players in the browser space are finding it difficult to keep pace with all the new APIs being introduced to the web these days. Even Microsoft sort of gave up and killed their browser engine in favor of a Chromium fork.
It's a tricky situation with no obvious solutions. The Pale Moon dev's plea to slow down isn't going to work; devs want these new features and they're not going to stop using them just because a smaller browser developer with no significant market share can't keep up.
There is, perhaps, the beginnings of a solution with the Extensible Web Manifesto[1], which aims to shift the focus of browsers from implementation of many high-level features to instead focus on smaller, simpler low-level features which high-level features can then be built on top of. (Though ironically, web components are a part of this movement.) CSS Houdini[2] is one such feature which seems like it might be relevant in this specific situation (though whether it's helpful I have no idea; in the short term I suspect it might just translate to a lot more work).
[1]: https://github.com/extensibleweb/manifesto
[2]: https://developer.mozilla.org/en-US/docs/Web/Houdini