<!doctype html>
<style>
fieldset:invalid {
  border: 10px solid red;
}
fieldset:valid {
  border: 10px solid green;
}
input.foo {
  color: green;
}
</style>
<div>
  <fieldset id="fieldset">
    <div id="ancestor">
      <input type="text" id="requiredInput" required>
      <input type="text" id="other">
    </div>
  </fieldset>
</div>
<script>
window.onload = function() {
  other.classList.add('foo');
  ancestor.remove();
};
</script>