WebGL Build Shader Error "Incompatible min precision type"

So I'm working on a shader and everything seems in order - works well in Editor and in a Windows build for example. However when I try and run it in a WebGL build I get errors when building. Specifically the following error multiple times (one for each multi-compile).

That comes once for most of the multi-compile versions of the shader.

I've tried googling the error but hadn't had much success. Before I go try and figure it out on my own I thought I'd see if someone here has any clue what to do.

 

After some help and experimentation we narrowed it down and I sent a bugreport:

https://fogbugz.unity3d.com/default.asp?934464_sjh4cs4ok77ne0cj

 

Well done!
It helps me a lot.