We use the emscripten compiler toolchain to cross-compile the Unity runtime code (written in C and C++) into WebAssembly (also known as Wasm), which has been designed to be small in size, load-time and memory efficient as well as able to deliver close to native speed execution. For more information about WebAssembly in Unity, check this blog post.
To convert the .NET game code (your C# scripts) into WebAssembly, we use a technology called IL2CPP. IL2CPP takes .NET bytecode and converts it to corresponding C++ source files, which is then compiled using emscripten to convert your scripts to Wasm.
Unity WebGL content is supported in the current versions of most major browsers on the desktop, however there are differences in the level of support offered by the different browsers. Mobile devices are not supported by Unity WebGL.
Not all features of Unity are available in WebGL builds, mostly due to constraints of the platform. Specifically:
System.Threading namespace is not supported.
WebGL builds cannot be debugged in Visual Studio. See: Debugging and trouble shooting WebGL builds.
Browsers do not allow direct access to IP sockets for networking, due to security concerns. See: WebGL Networking.
The WebGL graphics API is equivalent to OpenGL ES 2.0 and 3.0, which has some limitations. See: WebGL Graphics.
WebGL builds use a custom backend for Audio, based on the Web Audio API. This supports only basic audio functionality. See: Using Audio in WebGL.
WebGL is an AOT platform, so it does not allow dynamic generation of code using
System.Reflection.Emit. This is the same on all other IL2CPP platforms, iOS, and most consoles.
2018–03–19 Page amended with limited editorial review
MonoDevelop replaced by Visual Studio from 2018.1
WebAssembly replaced asm.js from 2018.2
asm.js removed in 2019.1