chromium/chrome/test/data/extensions/api_test/scripting/remove_css/frame3.html

<html>
  <style>
    #main {
      color: red;
    }
  </style>
  <div id="main">
    Hello.
  </div>
  <iframe id="frame4" srcdoc="<style>#main{color:red}</style><div id='main'>Hello.</div>"></iframe>
</html>