chromium/third_party/blink/web_tests/fast/dnd/resources/event-mouse-coordinates-nested-iframe.css

html, body {
  margin: 0;
  padding: 0;
  border: 0 none;
  overflow: visible;
  height: 100%;
}

iframe {
  margin: 0;
  padding: 0;
  border: none;

  position: absolute;
  left: 30px;
  top: 50px;
  overflow: visible;
}

iframe#iframe1 {
  width: 700px;
  height: 600px;
}
iframe#iframe2 {
  width: 650px;
  height: 550px;
}
iframe#iframe3 {
  width: 600px;
  height: 500px;
}