At one of the places I worked, a relatively large university-hospital, there was an entire Java-UI framework build based on C#s XAML. But it could also experime

At one of the places I worked, a relatively large university-hospital, there was an entire Java-UI framework build based on C#s XAML. But it could also experimentally be compiled to html/css/js. Writing any type of UI in Swing is just painful - but this system made it so much nicer. It also had a WYSIWYG editor that you could interop with actually writing the code. Plus you could write either the 'markup language' or the "Java" flavour directly. Making complex responsive UIs was a breeze. It's a shame it did not get open sourced - but it was tied into a great amount of internal tooling. (The entire codebase was 28+ million lines of code when I left). It's one of the few non-open source libraries I've worked with that really impressed me.