TC39 - Ecma Technical Committee 39
TC39 is a committee with members from companies that have deeply invested in web technologies, like Google, Mozilla and Apple as they have their browsers etc. Apart from them, there are more than dozens more from various organisations as well.
TC39's key job is to is to develop, maintain, and evolve the ECMAScript standard. They meet every other month and vote on proposals and review work. TC39 proposals go through 5 stages, starting from Stage 0 to Stage 4. Stage 0 is majorly well-formed ideas/plans whereas Stage 4 means the feature will be available as part of the next release. All proposals are open source and can be viewed by anyone and everyone is welcome to submit their ideas.
JS evolves rapidly yet it is always ensured to be completely backwards compatible i.e. old features are never removed and only new features are added. However, there can be issues when your program uses the latest feature but the user's browser is quite older. So this forwards-compatibility problem is solved by using transpilers like Babel. Assuming that you are using the 'const' keyword, Babel will generate a version of your program that uses 'var' to ensure that everything works as expected. Although a very simplified example, this holds for all such new features that aren't available widely yet, e.g. Optional Chaining.
In contrast, HTML and CSS are forwards-compatible, which means if you use a vintage HTML code and run it today, it might or might not work well. As HTML/CSS are formatting languages, the broken parts are just skipped by the browsers and continue to render the rest of the parts.