All ECMAScript is standardized because that refers to the standard.
I think you’re discounting the different environments that JS runs in. Something like C runs in a much more uniform environment (the OS) while JS must be able to run in different runtimes. It’s like how windows has APIs that C can access, but obviously you can’t access them when running C from other OSes (forgive me if that’s inaccurate, I don’t use C often).