Over at Microsoft's Modern.IE status page, many features are listed as being developed or considered. This includes support for Mozilla-developed ASM.js and, expected to be included in ECMAScript 7th edition, SIMD instructions. This is the one that I wanted to touch on most. SIMD, which is implemented as SSE, AVX, NEON, and other instruction sets, to perform many tasks in few, actual instructions. For browsers which support this, it could allow for significant speed-ups in vector-based tasks, such as manipulating colors, vertexes, and other data structures. Emscripten is in the process of integrating SIMD support and the technology is designed to support Web Workers, allowing SIMD-aware C and C++ code to be compiled into SIMD.JS and scale to multiple cores, if available, and they probably are these days.
In short, it will be possible to store and process colors, positions, forces, and other data structures as packed, 32-bit 4-vectors, rather than arbitrary objects with properties that must be manipulated individually. It increases computation throughput for significantly large datasets. This should make game developers happy, in particular.
Apparently, some level of support has been in Firefox Nightly for the last several versions. No about:config manipulation required, just call the appropriate function on window's SIMD subobject. Internet Explorer is considering it and Chromium is currently reviewing Intel's contribution.