title: Howto make RequiredFieldValidator change the css class of the parent div
body: <p>I have a TextBox input element that has a RequiredFieldValidator as such:</p>
  
  <pre><code>&lt;div&gt;
      &lt;label&gt;First name&lt;/label&gt;
      &lt;asp:RequiredFieldValidator ID="RequiredFieldValidatorFirstname" runat="server" ErrorMessage="Required" ControlToValidate="TextBoxFirstname"&gt;&lt;/asp:RequiredFieldValidator&gt;
      &lt;asp:TextBox ID="TextBoxFirstname" runat="server"&gt;&lt;/asp:TextBox&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>When the TextBox is empty on submit I want to add the class 'form-error' to the parent Div:</p>
  
  <pre><code>&lt;div class="form-error"&gt;
      &lt;label&gt;First name&lt;/label&gt;
      &lt;asp:RequiredFieldValidator ID="RequiredFieldValidatorFirstname" runat="server" ErrorMessage="Required" ControlToValidate="TextBoxFirstname"&gt;&lt;/asp:RequiredFieldValidator&gt;
      &lt;asp:TextBox ID="TextBoxFirstname" runat="server"&gt;&lt;/asp:TextBox&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Is this possible, and if so - how do I do it?</p>