<!DOCTYPE html>
<html>
  <body>
    <style>
      #a { border: 1px solid green; width: 150px; height: 200px; }
      #b { border: 1px solid black; min-width: 200px; height: 100px; }
    </style>
    <div id="a">
      <div id="b">
      </div>
    </div>
  </body>
</html>