The WebGL build option allows Unity to publish content as JavaScript programs using HTML5 and WebGL, which can then be run by supported web browsers. To build and test your content for WebGL, just choose the WebGL build target in the Build Player window, and hit Build & Run. Note that WebGL in Unity 5.0 is an early-access preview release and has some functionality limitations.
“Preview” means that Unity is capable of publishing content to WebGL, but there are some limitations in features and in browser compatibility. In particular, the following features are not supported:
In terms of browser support, this initial version only supports the desktop versions of Firefox and Chrome (other browsers might work for some content, but only these two are officially supported).
We expect to resolve most of those limitations (except for things which are restrictions imposed by the platform) during the 5.x release cycle, and to be able to support a wider range of browsers as well, as the platform matures – at which point we will drop the “preview” label and make WebGL a fully supported build platform in Unity.
To run in WebGL, all our code needs to be JavaScript. We use the emscripten compiler toolchain to cross-compile the Unity runtime code (written in C and C++) into asm.js JavaScript. asm.js is a very optimizable subset of JavaScript which allows JavaScript engines to AOT-compile asm.js code into very performant native code.
To convert the .NET game code (your C# and UnityScript scripts) into JavaScript, we use a technology which we call IL2CPP. IL2CPP takes .NET bytecode and converts it to corresponding C++ source files, which we then compile using emscripten to get your scripts converted to JavaScript. This conversion process might run into some compatibility issues with your script code in the early-access release, which we will work on resolving, but as long as you don’t require threads or dynamic code generation features it is expected to work.