Every GUI desktop client I'm aware of embeds a real rendering engine. The difficult one is Outlook, because Microsoft made a conscious decision that compatibility between Outlook and Word was more important than between Outlook and web browsers. I believe the Word/Outlook HTML engine was based on IE 5.5.
The MSO renderer is a buggy and incomplete implementation of HTML 3.2, and I think high-DPI support is the only change of any substance at all in the last twenty years.
