title: Attributes.Add("class", "className") but preserve existing class
body: <p>Simple thing, well, I think it is.</p>
  
  <p>I need to Add a class to an element within an asp:repeater under certain conditions, using VB.</p>
  
  <p>So, I can do</p>
  
  <pre><code>ITEMID.Attributes.Add("class", "classToAdd")
  </code></pre>
  
  <p>But this removes the existing classes and therefore screws up my CSS. </p>
  
  <pre><code>ITEMID.Attributes("class") = "classToAdd"
  </code></pre>
  
  <p>Seems to do the same thing.</p>
  
  <p>How do I <em>add</em> a class to an element, whilst preserving it's existing class values?</p>