WebGL in Internet Explorer 11
withinwindows.com
withinwindows.com
Hopefully they just happen to have HLSL support now because it was easy and are still working on GLSL. Otherwise it's not WebGL.
They can alternatively just require at least opengl es 2.0 support installed through gfx driver, thus only working on desktop machines. Not that it will make their plans of phone+tablet+desktop unification easier but, hey, it might shut up whining hipster developers for at least a few years until the realization that MS intentionally underdelivers dawns on them.
Ack, I hope GLSL shaders are well supported. It would be terrible to have to write a whole second set of shaders in HLSL for IE.
Unfortunately, complex renderings can produce some tough-to-debug problems when things get lost in translation. The good news is that ANGLE is still quite good, actively improving, and it allows automatic translation of GLSL to HLSL.