I've heard rumblings of SIMD coming to JS for awhile now, but I just now got around to watching this talk by Moh Haghighat from Intel. As I only had a basic understanding of SIMD (and vectorization in general), I found this a really enlightening peek into the tech.
Youtube:
In short, SIMD.js will not necessarily require custom code on the part of javascript programmers, as future JIT engines will automatically optimize incoming code to take advantage of SIMD where applicable. Of course, explicit use of SIMD functionality is coming first, as JIT integration will take more time.
Also, many mobile platforms already support SIMD via NEON, something I had no idea about.