We created a genuinely windowless branch of Chromium two years ago named Awesomium; it's free for non-commercial use and is used in several major games and commercial projects: http://www.awesomium.com
If you only run on Windows or OS X, it's windowless, but it's not headless.
Headless means something running on a command-line console, and you can't run either of those operating systems that way.