<form>
  <p>
    <label>
      Check the classes of the input and the error message in the developer
      tools of your browser.<br>
      <input type="text" name="foo" required>
    </label>
  </p>
  <p>
    <button>go</button>
  </p>
</form>

<script>
hyperform(window, {
  classes: {
    valid: 'yay',
    invalid: 'oooh',
    validated: 'first!',
    warning: 'shoot why is there a warning?',
  }
});
</script>