The attribute action="" (empty string) on the <form> element is invalid
HTML5. The spec (https://html.spec.whatwg.org/#attr-fs-action) says:
> The action and formaction content attributes, if specified, must have
> a value that is a valid non-empty URL potentially surrounded by
> spaces.
Emphasis on non-empty. The action attribute is allowed to be omitted, in
which case the current URL is used which is the same behavior as now.