It's interesting to get other views on this.
HTML/CSS is very flexible and allows the ability to do a ton of things. Millions of people also know it so finding resources to work with it is very easy and cheap. There are also thousands of libraries already available ready to use where you can see the source for all of them.
I don't want to have a crappy HTML renderer bloating my game code.
I think this is debatable on it bloating your code itself. When making a game you'll have a lot of libraries "bloating" your code so not sure why you would see this as any different. If anything the separation has many benefits.
What exactly is the advantage of using HTML over other options? What does it gain me over, say, an xml driven windowing/button system?
Some of the things I've stated above show some advantages but for me the biggest one is that HTML/CSS is well established. It's not going anywhere anytime soon and will evolve for decades to come. It has a huge backing and following where as any custom xml driven windowing system just doesn't have that. Even xaml which was created by MS is much more likely to die out over HTML/CSS in the future. On top of that all rendering is done for you. Maybe some people enjoy making GUI's, but the bottom line is if you are trying to make a game and the gameplay is what matters to you the most, you just need a GUI that is reliable, flexible, has longevity, and has a ton of backing. To me HTML/CSS rules in all of those spaces. That's my view on it.
http://www.awesomium.com/ looks interesting. I'll have to dig deeper to see if it's as bad as librocket though.
Modern HTML/CSS is quite resource intensive which makes it unsuitable for game GUIs
What seems interesting with awesomium is that it seems to run on it's own process so should have no impact on our games process. That would for sure be required. Also, I think we'd have to look at what is really in a game gui. They aren't exactly all that complicated the majority of the time. I know some games try to get cute with them but most games are pretty easy going with the gui itself. I wonder how taxing an average game gui would really be.