Duetto is an alternative open-source project to EmScripten
A developer for the company has now shared that Duetto now has a fully working libc and libc++ implementation for a standard C and C++ library, CMake/Autotool support has been integrated, and there's now full support of the DOM and HTML5 APIs. This does include use of WebGL from C++. The company has also made a small WebGL demo that utilizes the Bullet physics library.
More details on the Duetto project can be found in this blog post
. The company plans to release Duetto as open-source for non-commercial use while a commercial product will also be available in the coming months.