WebAssembly is the new cross-browser attempt to create a performant and secure low-level language for the web. It’s still brand-spanking new, but we can only expect the number of stories about wasm to increase. A search for the term WebAssembly shows 59 discussions to date.
Given that it’s an extension of asm.js and JavaScript, that seems like the right tag for now? Or maybe just the web tag?
Usually the way suggestions work is you list all the stories that have been posted which would get the new tag suggestion, so you can show there is a market for such a tag.