What C++ standard version is supported by Emscripten for webassembly?

On the website it is said you can compile C/C++, and that the toolchain uses Clang.
This also supports C++ libraries such as Qt, DSL, or OpenGL.
https://emscripten.org/docs/introducing_emscripten/about_emscripten.html

But they do not specify if C++11, 14 or more are supported. Nor the Clang version used by the toolchain.

How can I know what C++ version is acceptable (i.e. compiling well, and tested)?