WebAssembly is a new type of code that can be run in modern web browsers — it is a low-level assembly-like language with a compact binary format that runs with near-native performance and provides languages such as C/C++ with a compilation target so that they can run on the web. It is also designed to run alongside JavaScript, allowing both to work together - developer.mozilla.org 
YOUTUBE TwuIRcpeUWE This is a video of Epic's "Zen Garden" demo running in Firefox. The demo is built with WebAssembly and WebGL 2, both emerging standards that enable amazing video games and applications in the browser.
__WebAssembly__ (Wasm, __WA__) is a web standard that defines a binary format and a corresponding assembly language-like text format for executable Computer code in World Wide Web pages.
- Languages that can compile to wasm - github
It is meant to enable executing code nearly as quickly as running native machine code. It was envisioned to complement JavaScript to speed up performance-critical parts of web applications and later on to enable web development in languages other than JavaScript.
WebAssembly does not attempt to replace JavaScript, but to complement it. It is developed at the World Wide Web Consortium (W3C) with engineers from Mozilla, Microsoft, Google, and Apple Inc. - wikipedia
YOUTUBE DKHuEkmsx3M Watch this presentation to learn about the recent updates on the state of WebAssembly and view a short demo of Rust programming language interacting with the land of browsers.
It is executed in a Sandbox (computer security) in the web browser after a verification step. Programs can be compiled from high-level languages into Wasm modules and loaded as library (computing) from within JavaScript applets.
YOUTUBE HktWin_LPf4 Lin Clark: A Cartoon Intro to WebAssembly | JSConf EU 2017
YOUTUBE 6Y3W94_8scw WebAssembly (aka wasm) is shipping in all modern browsers, but since it's (intentionally) very low level, it can be difficult to understand how it will be critical to future web applications--and definitely not just games and C++!
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Now, starting with Firefox 52, and coming very soon in Chrome, WebAssembly allows you to compile C or C++ to the Web while maintaining near-native performance. A whole new set of developers are now able to target the Web in ways that used to be impossible--and without using plugins - mzl.la
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# See also
- Webflow - p5.js - OpenFrameworks - Unity 3D - Simulation - Permanent Web