LibRocket
libRocket is the C++ user interface middleware package based on the HTML and CSS standards. It is designed as a complete solution for any project's interface needs.
libRocket uses the time-tested open standards XHTML1.0 and CSS2.0 (while borrowing features from HTML5 and CSS3), and extends them with features suited towards real-time applications. Because of this, you don't have to learn a whole new proprietary technology like other libraries in this space.