title: What are "data-url" and "data-key" attributes of <a> tag?
body: <p>I've faced two strange attributes of an html tag <strong></strong>. They are  <strong>"data-url"</strong> and <strong>"data-key"</strong>.</p>
  
  <p>What are they and how can they be used?</p>
  
  <p>For some reasons i can't show the exact example of the HTML file I've found them in, but here are some examples from the web with such tags:</p>
  
  <ol>
  <li><a href="https://github.com/clarkni5/calchd/blob/master/index.html" rel="noreferrer">data-key</a> </li>
  <li><a href="https://github.com/prose/prose/blob/master/templates/toolbar.html" rel="noreferrer">data-key</a></li>
  <li><a href="https://gist.github.com/JoshLedgard/4947561/raw/9a848bd9b7f16797eeb7b52ba5f90dcd1ff077d9/KOL_V1_Widget_HTML_Result.html" rel="noreferrer">data-url</a></li>
  </ol>
  
  <p>PS: I've tried to Google, but no useful results were found. </p>