<!DOCTYPE html>
<div style="width:300px; height:200px; border:5px solid black; border-radius:30px; overflow:hidden;">
  <div style="position:relative; left:100px; top:100px;width:10px; height:10px; background:red; will-change:opacity;">
    <div style="width:500px; height:500px; background:green; will-change:opacity;">
    </div>
  </div>
</div>