Html validation is actually quite powerful and can help with most "quality of life" validations. You know the "this field is required" or "this field needs to be a number" kind of thing.

Even more, html validation has by far a much better UX than most custom made JS solutions. For example if there is an error in the validation, your cursor is immediately transported to the offending field where you can do the correction, with the appropriate css classes available to highlight the field properly.

And for the stuff that html validation is not powerful enough, well then you most probably want to check that on the server as well anyway.