And I have to disagree with you. Doing UIs in native frameworks like JavaFX or even in a pure C++/WinAPI, is much more saner than doing any kind of UI in HTML/CSS. Ignoring flexbox for a second, which is kind of bleeding edge new thing, remind me - how exactly do you vertically center stuff with CSS? :P.

My two main issues with doing UIs on the web are:

- web stack is terribly unpredicatable; you're trying to force an inherently document-layout-oriented model into a free-form 2D canvas, which leads to layouts exploding whenever user switches browser/OS/device/timezone/whatever.

- the code makes no sense; no matter how crappy the Java APIs were (due to deficiencies of the language mostly), the code you wrote at least did what it said, because it expressed the UI concepts explicitly. HTML/CSS is a huge mess of insanity when it comes to doing UI work. I mean, in what reasonable world should the idea "center this" be expressed as "set the left and right margin to 'auto'"? CSS is full of stuff like that.

(And then there are some silly philosophies like "separation of form and content" which everyone touts, but rarely anyone actually follows; see e.g. grid layouts.)

The difference between what you are doing and what desktop UI has always enjoyed is a component system that works flawlessly. The Web Components specification is a good step forward in this regard, but as many people have pointer out, try to do something trivial like a modal wizard in Java/.NET and try the same thing on the web side.