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  <meta charset="UTF-8">
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  <title>Device light (ambient light sensor) experiment by Will Murray</title>

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    body {
      text-align: center;
      background-color: black;
      color: white;
    }
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  <div id="devicelightValue">0</div>

  <a href="https://developer.mozilla.org/en-US/docs/Web/API/DeviceLightEvent"
     target="_blank"
     style="position: absolute; top: 1em; right: 1em; opacity: .4; color: white">
    <small>More info & compatibility</small>
  </a>

  <script>
    window.addEventListener('devicelight', function (event) {

      var html = document.getElementsByTagName('html')[0];
      var devicelightValue = document.getElementById('devicelightValue')

      devicelightValue.innerText = event.value
      devicelightValue.style.fontSize = (event.value + 10) + 'px'

    })
  </script>

</body>

</html>