title: CSS child height 100% with parent padding
body: <p>I've gone through a lot of similarly named questions, but have not been able to find an answer.</p>
  
  <p>When a parent div has a padding and I create a child div, width 100% follows the content-box size of the parent, but height 100% seems to take the border box value of the parent.
  So for instance a parent with height 100px and 5px padding would create a child of height 100% with the height 100px instead of 90px.</p>
  
  <p>Can anyone help me achieve the height of the parents content-box with padding?</p>
  
  <p>I want to achieve a nice horizontal border, that is clipped in both ends, but is still centered.</p>