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chromium/third_party/blink/web_tests/external/wpt/html/editing/dnd/draggable-areas/border-radius.html 

<!DOCTYPe html>
<meta charset='utf-8'>
<title>drag and drop – draggable area boundaries, border-radius</title>
<style>
a {
  display: block;
  height: 200px;
  width: 200px;
  background-color: blue;
  border-radius: 100px;
}
div {
  border: 1px solid black;
  height: 200px;
  width: 200px;
}
</style>

<ol>
 <li>Try dragging the white area within the black square, outside the blue
 circle. It should <em>not</em> be draggable.</li>
 <li>Drag the blue circle below. It should be draggable.</li>
</ol>

<div><a draggable="true" ondragstart="event.dataTransfer.effectAllowed ='copy'"></a></div>