title: make a input field required if radio is checked
body: <p>Can I somehow insert the required attribute into an input field only if a certain radio is checked?</p>
  
  <p>I have an input field named "ladder-meters" that is not required to be filled per default. But if the user checks a radio button that is named "ladder" the "ladder-meters" field are required to be filled.</p>
  
  <p>My input looks like this:</p>
  
  <pre><code>&lt;input type="text" name="ladder-meters" id="ladder-meters"&gt;
  </code></pre>
  
  <p>and should like this at the onchange event on the radio</p>
  
  <pre><code>&lt;input type="text" name="ladder-meters" id="ladder-meters" required&gt;
  </code></pre>