Compare C++ standardization. Most people are using two year old compilers. While the newest compilers do implement draft features, these features are almost all standardized in the next ~3 years. The standards are clearly versioned and you can put newer compilers into modes to check against older standards if your code needs to compiler on multiple compilers. Features are never removed.

By contrast, on the web, everybody is running brand spanking new browsers. Experimental technologies are sometimes implemented before full draft standards are even written. Many or most drafts that are written never become standards. If they do become standards, it's often a decade or more away. Alternatively the living standards of what browsers actually support aren't versioned. It's not possible to put a browser into JavaScript 99 mode - the easiest way to check your code for conformance is to automate your test cases across a variety of browsers. And, features are sometimes removed because they are deemed security or privacy concerns, so conforming to an old standard is not sufficient to ensure proper functionality under modern browsers.