2019.3 Editor GUI

Apparently the way the UI is wrote in 2019.3 has changed, is this also relevant to the Editor GUI? And if so is there any articles, any where that would explain anything about it!
Ive heard around that there’s CSS elements now somewhere and quite a few things have changed or are going to be changing from the way the old system worked, and of course I cant find really any material from Unity on it, because it’s probably still in preview.
Can someone point me in the right direction, is the way Editor GUI stuff changing at all? I would like to be working on a new tool right now but I’d rather not write it in a soon to be deprecated structure.

I’m not sure if this should go here, I’ve been searching for nearly 6 hours now for an answer. Please bear with me.

I haven’t used it yet, but it’s called UIElements

The plan is to have a single GUI solution for creating both Editor and Runtime GUI’s.
AFAIK it doesn’t yet officially support Runtime GUI, but it supports Editor GUI.
They have shown a preview of it working in Playmode as well, though.

Here is the dedicated forum

Thankkkk you so much, I kept running into older stuff and now I can finally at least move forward, very much appreciated!!!

Here’s some useful pages: