title: why is $(document).height() different from $(document.body).height()?
body: <p>I have been googling for a while, but still could not find why<br>
  is <code>$(document).height()</code> different from <code>$(document.body).height()</code>?  </p>
  
  <p>What's more, could anyone tell me the corresponding relation between the height methods<br>
  in jQuery and the raw JavaScript ones?  </p>
  
  <p>For example, <code>$(document).height()</code> is equivalent to <code>document.documentElement.scrollHeight</code>.  </p>