Re: WYSIWYG CSS, we initially started w/ flexbox but things like the difference between margins, padding, and absolute positioning confused tons of designers coming from Photoshop/Sketch.
We ended up implementing our own layout system, which can be used directly in production (if you want to ship a runtime), otherwise it's possible to map layouts to something like flexbox+grid or native engines if your designers stay away from things that are hard to implement on those platforms (e.g., aspect ratio).
Our goal is to help people think about layout directly, not try to hide the inherent complexity from them. At the end of the day, most all digital products require close collaboration between designers and implementors.