title: Why do negative margins affect my page width?
body: <p>Please reference the following <a href="http://jsfiddle.net/wdm954/Fcznp/9/" rel="nofollow">example</a>:</p>
  
  <p>In it, an outer div 200px wide is meant to establish our page width.  It contains an inner div 400px wide, but with left/right negative margins of -100px.  </p>
  
  <p>My intended end result is that the  browser register total content width at 200px, not 400px, but horizontal scrollbars show up as soon as the window is resized to less than 400px.  What is going on here?</p>