THat's not right, the same is true of JS. You'd run a "build" step on a system you control to transpile your fancy modern JS back as far as you'd like official

THat's not right, the same is true of JS. You'd run a "build" step on a system you control to transpile your fancy modern JS back as far as you'd like official support and put the output JS file on your webserver. Using modern JS features does not require universal support.