Unfortunately, it doesn't work with WebGL as of Unity 5.3.4.
http://docs.unity3d.com/Manual/webgl-performance.html
"Currently, the JavaScript language does neither support multi-threading, nor SIMD. So, any code which benefits from these features will see bigger slowdowns then other code. You cannot write threading or SIMD code in WebGL in your scripts, but some engine parts are normally multi-threaded or SIMD optimized, and will run less performant on WebGL because of this. An example is the skinning code, which is both multi-threaded and SIMD-optimized. You can use the new timeline profiler in Unity to see how Unity distributes work to different threads on non-WebGL platforms. Longer term, we are hoping that these features will become available on WebGL as well."
It builds without errors (API Compatibility Level must be changed from .NET 2.0 subset to .NET 2.0) but when it runs, I get an error in the browser.
http://answers.unity3d.com/questions/1115818/html5-missing-function-pthread-create.html
However, would it be possible to isolate only the single threaded dispatchers (e.g., fibers, coroutines) so we can still use it with WebGL?