This request header contains the [name] of a form field being validated.
When seeing this header, the server is expected to validate (but not save)
the form submission and render a new copy of the form with validation errors.
See the documentation for input[up-validate] for more information
on how server-side validation works in Unpoly.
The server is free to respond with any HTTP status code, regardless of the validation result.
Unpoly will always consider a validation request to be successful, even if the
server responds with a non-200 status code. This is in contrast to regular form submissions,
where a non-200 status code will often update a different element.
Example
Assume we have an auto-validating form field:
<fieldset>
<input name="email" up-validate>
</fieldset>
When the input is changed, Unpoly will submit the form with an additional header:
X-Up-Validate: email
