The World Wide Web Consortium (W3C), an international community built to develop web standards, formed the WebAssembly Working Group in April, 2015, to standardize WebAssembly and oversee the specification and proposal process. Since then, the Core Specification and corresponding JavaScript API and Web API have been released. The initial implementation of WebAssembly support in browsers was based on the feature set of asm.js. WebAssembly's binary format and corresponding .wasm file combined facets of asm.js output with PNaCl's concept of a distributed executable.
So how will WebAssembly succeed where NaCl failed? According to Dr. Axel Rauschmayer, there are three reasons detailed at http://2ality.com/2015/06/web-assembly.html#what-is-different-this-time:
Second, the interoperability with the web platform and JavaScript is excellent. Using WebAssembly code from JavaScript will be as simple as importing a module.
Third, this is not about replacing JavaScript engines, it is more about adding a new feature to them. That greatly reduces the amount of work to implement WebAssembly and should help with getting the support of the web development community."