There's a reason subsets like asm.js are created to make the Web a viable target:
Wt has tight integration with several modules, like JSON, or WebGL: http://www.webtoolkit.eu/wt/doc/reference/html/namespaceWt.html
If you have some prior experience with Qt and OpenGL, you should find a familiar ground with Wt and WebGL.
OP: as for Emscripten, see these two links, should explain most:
// the last one links to this: http://slideshare.net/andreweissflog3/quovadis2013-cpp-ontheweb