There are a lot of great UI frameworks for MonoGame. However, I really wanted to be able to take advantage of my existing knowledge of HTML/CSS, so I explored the possibility of using an offscreen instance of Chromium Embedded to render my front-end UI.
As it turns out, this technique works pretty well. I have an example repo here: https://github.com/fr0/MonoCef.
Feel free to copy it or steal from it; I'm just posting this in the hopes that someone might benefit from my work.