You absolutely have to do server-side validation anyway, since clients can lie. Since you're doing the server-side validation you have to do anyway, it's child

You absolutely have to do server-side validation anyway, since clients can lie. Since you're doing the server-side validation you have to do anyway, it's child's play to return the HTML form with the values the client submitted pre-filled. Client-side validation is a user convenience; it's awesome, but not sufficient in itself, because it can only protect the client, not the server, from invalid values.