We have an in-house UI library, mainly because we started writing code in the mid-2010s and there was nothing good out there at the time. Maybe that's changed but while I haven't researched it thoroughly, I also haven't come across anything that looks like it does everything ours does. But our UI lib aims to be as simple and straightforward as possible, standard model-view-controller, no virtual DOM, making full use of HTML and CSS features. Personally I view a lot of modern CSS frameworks with suspicion as they all seem to massively overengineer this stuff. We have a whole desktop IDE running on basically old fashioned desktop app engineering style but on top of HTML, and it works great.
Apart from a few extra services on the side, the main app is all static resources so no real backend to speak of. Again, engineered like a traditional desktop app with everything running locally, which also means the UI always responds promptly (no waiting for network) and it all works offline.
Maybe I should write up a blog post about how it all works, but not sure how many people out there want to hear more!