border-shape-shadow-transparent-ref.html

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<!DOCTYPE html>

<html>

<title>CSS Borders: border-shape with transparent background ref</title>

<style>

  body { padding: 50px; }

  #target {

    width: 100px; height: 100px;

    background: transparent;

    border-radius: 50px;

    box-shadow: 0 0 0 20px black;

</style>

<body>

  <div id="target"></div>

</body>

</html>