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

html, body {
  margin: 0;
  padding: 0;
  display: block;
}
.box {
  display: block;
  border: 1px solid black;
  width: 100px;
  height: 100px;
}
.dragged {
  position: absolute;
  left: 199px;  /* Get the box's client area at 200,100. */
  top: 99px;
  cursor: pointer;
}
.dropzone {
  position: absolute;
  left: 399px; /* Get the box's client area at 400,300. */
  top: 299px;
  border-style: dashed;
}