JS form validation quite significantly predates HTML form validation. The alternative was server side form validation, for the longest time. HTML form validatio

JS form validation quite significantly predates HTML form validation. The alternative was server side form validation, for the longest time. HTML form validation was meant to standardise and reduce the need for JS validation, but for UX reasons it hasn't really managed that. And that you'll need JS anyway if you're doing validation specific to your business logic that doesn't fit into the set of rules HTML validation has.