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

<!DOCTYPE html>
<script src="resources/srcset-helper.js"></script>
<style>
  #standard {
    background-image: image-set(
      url('resources/deleteButton.png') 1x,
      url('resources/deleteButton-2x.png') 2x
    );
  }

  #prefixed {
    background-image: -webkit-image-set(
      url('resources/deleteButton.png') 1x,
      url('resources/deleteButton-2x.png') 2x
    );
  }

  .foo {
    width: 90px;
    height: 90px;
    background-repeat: repeat;
    background-size: 30px 30px;
  }
</style>

<div>
  This test passes if the divs below contains 9 evenly spaced delete buttons,
  and if the high resolution resource is used when the deviceScaleFactor is 2.
</div>
<br>
<div id="standard" class="foo"></div>
<br>
<div id="prefixed" class="foo"></div>