title: Why does this button still has SPACE on top after setting margin-top to ZERO?
body: <p>The example can be found at  <a href="http://www.bootply.com/143jVqzP0A#" rel="nofollow noreferrer">http://www.bootply.com/143jVqzP0A#</a></p>
  
  <p>The button looks like this </p>
  
  <p><img src="https://i.stack.imgur.com/IIgs8.png" alt="picture"></p>
  
  <p>which has margin at top, though I set <code>margin-top</code> to 0. And I want  the button to fill the whole block vertically, which has no margin or space between the button and its parent block.</p>
  
  <p>Does anyone have ideas about this? Thanks!</p>