Comment by BrendanEich

JS was ubiquitous when DHTML was pushed by Microsoft, because DHTML required JScript aka JS.

CSS was not there in the '90s because Netscape didn't implement, and MS did its own subset.

JS and CSS both suffered from wildly inconsistent support between Netscape and IE, but JS at root had interop enough to support hotmail, later OddPost, much more. CSS had no extension mechanism based on JS then, so it suffered browser-specific support that was IMHO worse than JS suffered. No way to polyfill.