As the World Wide Web was gaining popularity in the mid-nineties, browser manufacturers started experimenting with interpreting program scripts embedded within webpages. By far the most successful of these was JavaScript, initally developed by Brandon Eich for Netscape.
Brandon Eich was hired to integrate the Scheme programming langauge into the Netscape browser. But when Netscape cut a deal with Sun Microsystems to bring Java Applets to their browser, his mission was altered to create a more Java-like langauge. He developed a prototype in only ten days, that blended Java syntax, the Self object-orientation approach, and Scheme functionality.
Netscape eventually submitted JavaScript to ECMA International, resulting in the ECMAScript standard, and opening the door for other browsers to adopt JavaScript. Currently all major browsers support the full ECMAScript 5 standard, and large chunks of ECMAScript 6 and some parts of later versions as well. Moreover, transpilation can be utilized to make newer ECMAScript-compliant code run on older browser versions.
The ECMA standard is maintained by ECMA International, not the W3C. However, its development process is very similar, involving stakeholders developing proposals for new and improved features.