Typically for fetch() I return the error as "text/plain" with the appropriate status code. It is still necessary to give a preflight validation in Vanilla JS, apply an error style and helpful feedback.

HTML has come a long way since the bad old days. General constraints can be applied directly to the input element. It still makes sense add additional explanations for the invalid input with JS though.

https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/...