You also have to implement your own validation on the backend anyway. There's always going to be someone trying to fiddle around with your form, either using some weird browser, curl, or some other tool that doesn't have the same form validation built in. You're not going to trust that the client actually did the validation, or did it correctly, so the backend still needs to be able to validate input and show the form, with validation errors, on all fields.

Frontend validation is only there to be helpful for the user, but if you can style it, or trigger it from the backend on submit, you have to implement your own styling anyway.