title: why use role attribute in html5
body: <p>I have created a web page using html5.</p>
  
  <p>I used source code from bootstrap.</p>
  
  <p>For example: <code>role="main"</code>, <code>role ="navigation"</code>, <code>role = "document"</code> etc.</p>
  
  <p>If I remove these attribute from my code, page didn't any change.</p>
  
  <p>So I want to know: what is the purpose of "role" attribute?</p>
  
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