C has to be recompiled per platform.

The advantage to something like using WebAssembly is you could write a program in C, compile it to WebAssembly, and then it could be run on anything that supports WebAssembly.

This could enable a future where you write one program that can run in a web browser, as a phone app, as a desktop program. All with only tweaking the UI.

Additionally the divide between a “web app” and a desktop app from a user perspective increasingly comes down to: do I need this to run fast (as a native program), or do I want it to be convenient to use (as a web app)? WebAssembly may very well become fast enough that it is fast and convenient for all but the absolute most demanding of applications.

WebAssembly's closest analog would be LLVM's IR, which many compilers like clang (for C/C++) or rustc output so that they can use LLVM's existing codegen. In WebAsm's case, however, the backend would be the different browsers and their sandboxes.

The C ABI describes a common calling convention and a few other details to allow different compilers to use each other's binaries, but C is hardly a common language in the way WebAssembly is meant to be.

Binary compatibility across architectures, no undefined behaviours are the primary ones.

Nasal demons