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chromium/third_party/blink/web_tests/fast/hidpi/image-set-content.html 

<!DOCTYPE html>
<script src="resources/srcset-helper.js"></script>
<style>
  #standard {
    content: image-set(
      url('resources/blue-100-px-square.png') 1x,
      url('resources/green-400-px-square.png') 2x
    );
  }

  #prefixed {
    content: -webkit-image-set(
      url('resources/blue-100-px-square.png') 1x,
      url('resources/green-400-px-square.png') 2x
    );
  }
</style>

<div>
  This test passes if the divs below are
  100px blue squares when the deviceScaleFactor is 1,
  and 100px green squares when the deviceScaleFactor is 2.
</div>
<br>
<div id="standard"></div>
<br>
<div id="prefixed"></div>