People need way more validations than html5 elements can provide.
They need CC number validation. Validations that depend on the content of sibling elements. And they need it to display in custom ways like internationalized or whatever.
Attach event listeners and handle those parts with JS. For internationalization, template on the server side, sending a different bundle for user agents in different countries. CSS is enough to fill the gaps with other internationalization concerns like rtl.
The platform provides enough to do forms. It's one of the few things the web is good at. It has been around for ages and works. I mean, what I'm using to type this is a plain HTML form. It doesn't need a massive JavaScript framework that heats up my CPU on every keystroke.
