Is anyone using the bootstrap UI css in production in 4.6?
This was part of a google summer of code. This requires change in the core and it wasn't "production ready" and hasn't been merged. For what I recall, they were a few tweaks that were "big gain", like automatically generate the input fields (most of?) so they use the same html structure and classes than bootstrap.

It would be great to complete the project and get it back in the core, but so far, no one picked it up (hint hint...)

What I often do is to use bootstrap and webform for nearly all the public facing forms, and it works fine

X+