Note that this site lists WebGL as "IE11+". That's false. IE11 has a flavor of webgl that's experimental (prefixed or not) that's reported as version WebGL 0.92 (this is basically an invalid specification conformant string, it's either 1.0 or something not done).
Huge gaps and bugs remain in Microsofts WebGL implementation which make it nearly impossible to use except for specific select usecases that Microsoft optimized for.
It took google and mozilla about 4 years to get a good WebGL implementation (and they're still not done). It'll take Microsoft years to come to bring their implementation on par with the rest of the WebGL world.
We shipped an update to the IE11 WebGL implementation to developers today as part of Windows 8.1 Update. We will roll this out to all IE11 users through Windows Update starting next week. There will be further updates to our WebGL implementation in the summer.
If there are specific use cases that you're interested in support for, please let us know what they are so that we can prioritise the order of our implementation.
Another measure that's also very useful is to run the webgl performance regression test suite every day to see if performance got worse or better with the changes.
Unfortunately there isn't a comprehensive GLSL syntax test suite, but GLSL has been much of a sore point in IE where some syntax that's valid GLSL would work except in IE (such as uniforms separated by a comma).
I've submitted some tickets to IE (and added more conformance tests to cover them) for some of the gaps (gl.SAMPLES, gl.STENCIL_BITS, gl.SUBPIXEL_BITS).
A thing that's also a sore point is IEs lack of support for very common extensions such as OES_texture_float_linear, WEBGL_compressed_texture_s3tc, WEBGL_depth_texture, OES_standard_derivatives, OES_vertex_array_object, ANGLE_instanced_arrays, OES_element_index_uint, WEBGL_lose_context. You can get an overview of the state of support on http://webglstats.com/
A note on floating point texture extensions. If you implement one extension (for instance OES_texture_float) you should really implement the companion extensions as well for texture_float_linear and color_buffer_float. Only the triplet of extensions provides comprehensive overview of support.
I think the demos above are fairly good usecases for gaps that you might have, because they exercise a lot of functionality, they're not bound to some specific framework (like three.js) but they are WebGL conformant.
Huge gaps and bugs remain in Microsofts WebGL implementation which make it nearly impossible to use except for specific select usecases that Microsoft optimized for.
It took google and mozilla about 4 years to get a good WebGL implementation (and they're still not done). It'll take Microsoft years to come to bring their implementation on par with the rest of the WebGL world.