I just don't think browsers are in a good position to implement advanced features like this, though. They are too general of a tool to be opinionated and if you aren't going to be opinionated I don't see how javascript isn't the answer.

What kind of autocomplete do you want? Which algorithm? What's the threshold if you're using some kind of "string distance" metric? What do you want to do style wise when you match portions of strings? What optimizations should be made for your specific case?

I don't see how a web browser is in a better position to make these types of features compared to a javascript library / web assembly.