This means that different browsers will support different features at any given point in time.
Websites like caniuse.com help us understand language implementation inconsistencies across different browsers.
A Quick History
In 1996 Netscape submitted the programming language to a standards creation organization called ECMA International in hopes to create a standard on how to use a language across different browsers and that standard named ECMAScript was created and released the next year, 1997.
There are two parties that contribute to updating and maintaining ECMAscript, TC39 and The open source community.
The Open Source Community
Anyone can create a proposal and a TC39 community member can champion a proposal, who will support and try to get that proposal into the latest version of ECMAScript.
A standards creation organization in charge of all sorts of different standards.
A specification for a programming language, think of it as a text reference that describes how a language should work, and not a programming language itself.
Technical Committee Number 39 is in charge of creating, maintaining, and finalizing ECMAScript proposals that will get included in newer versions of ECMAScript.
40-60 delegates meet six times each year to discuss proposed changes to the spec.
This is just one of the many sub-committees that reside under the ECMA International organization.