The high-level goal of the WebAssembly SIMD proposal is to introduce vector operations to the WebAssembly Specification, in a way that guarantees portable performance. post 
The set of SIMD instructions is large, and varied across architectures. The set of operations included in the WebAssembly SIMD proposal consist of operations that are well supported on a wide variety of platforms, and are proven to be performant. To this end, the current proposal is limited to standardizing Fixed-Width 128-bit SIMD operations.
This proposal derived many elements from the SIMD.js proposal, which in turn was originally derived from the Dart SIMD specification. The SIMD.js proposal was an API proposed at TC39 with new types and functions for performing SIMD computations, but this was archived in favor of supporting SIMD operations more transparently in WebAssembly. The WebAssembly SIMD proposal was introduced as a way for browsers to take advantage of the data level parallelism using the underlying hardware.