title: What does a colon mean within an HTML id attribute?
body: <p>I have seen an HTML code like this for the ID attribute:</p>
  
  <pre><code>id="t1:c3"
  </code></pre>
  
  <p>Can someone explain this to me? What is the purpose of the colon (<code>:</code>)?</p>
  
  <p>Thank you.</p>