title: How to make a Textbox required IF a Checkbox is checked
body: <p>How can I make a textbox required if a checkbox is checked?</p>
  
  <p>I figure I could write a custom validator, but I was hoping to avoid a full post back to check the validation if possible... I was thinking AJAX had something built in for this scenario, but I've been unable to find it. I'm thinking straight JavaScript would also be a solution, but I could use a head start if that's the best approach.</p>