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What do you mean there's no real benefit?

You give the same error messages based on whatever custom validation and control the output.

I've found it more annoying to mess with the browser validation API and using setCustomValidity/reportValidity etc. than to just use other validation libs. Ideally I'd use whatever library I want and they would call setCustomValidity for me though.

Using the related pseudo classes :valid, :invalid, :required, :optional is nice, but until last year you still had to do custom logic there because :user-valid/:user-invalid weren't implemented outside of Firefox. That created additional work and was annoying.

Do you have any examples?

Really? In my experience they easily cover the majority of cases - strings versus numbers, min and max length, upper and lower bounds to values, specifying what decimal level you’ll accept, you can even straight up provide a regex in the ‘pattern’ attribute.