For years, I (Jose) have been planning for a project like this. I prepared my self quite well. I went through the entire CSS Level 2 specification along with the HTML 4.01 specification.
One of the most interesting things I found is this: Drawing HTML is no more than laying out a bunch of boxes with borders margins and padding's. Once you overpass this paradigm, everything else is to help the code actually place the boxes on the right place, and then paint the string each box contains.
Imagine the power that drawing full-rich-formatted HTML on your controls can give to your applications. Use bold when you need it, italics on every message, and borders and fonts as you may like or need everywhere on the desktop application. One of the first projects where I will use it is on the tooltips of my Ribbon Project.
Although I have not tested it on mono yet, there should be no problem at all, since all of the code on the library is managed code and the methods it use to paint are quite basic. It draws lines, rectangles, curves and text.
In October 2012 I (Arthur) was looking to replace the usage of WinForms WebBrowser control by something that can render complex html and have good performance and stability characteristics. Obviously I was looking for fully managed solution preferably one that I will have full control over.
HTML Renderer project showed great promise but had significant performance issues, lacked many features (primary text selection) and wasn't updated for more than 3 years. Realizing there is no alternative I embraced the project making it my baby.