WebAssembly will be supported by all browsers and JS engines, but it's not something you would use as your source code. I think at the moment it's more focused on high-performance use cases, e.g. rewriting a CPU-intensive function in C++, compiling it to WebAssembly, and calling it from your JS code. Or taking an existing C++ codebase and running it on the web without really using the DOM. But in the long run, I think WebAssembly will make it possible to do things like DOM interactions in other programming languages.