Belgian Microsoft Student Partner François Remy recently poked around inside the build and discovered, among many things, interfaces for the cross-platform 3D graphics API. Unfortunately, Remy failed to get any WebGL content running to test the functionality, but another intrepid user discovered that a registry hack was all that was needed to make it operational.
Even then, it's not a complete implementation, as it only supports a subset of High Level Shader Language (HLSL) called IESL, which means it can't run a majority of WebGL content currently available.
Whether Microsoft will ship IE11 with WebGL support is unknown, but the fact it's working on building it into the browser makes total sense. I doubt it can convince the other major browser vendors to take up Direct3D -- despite Firefox support DirectWrite for font rendering -- and forcing developers to code up their content with both WebGL and Direct3D paths would be an even bigger ask.
Internet Explorer 11's leaked build (new features) [François REMY, via CNET]