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

<!DOCTYPE html>
<!--
  Tests that '-webkit-' prefixed image-set value is used
  when standard image-set fails parsing.
-->

<script src="resources/srcset-helper.js"></script>
<style>
  #foo {
    width: 100px;
    height: 100px;
    content: -webkit-image-set(
      url('resources/red-100-px-square.png') 1x,
      url('resources/red-100-px-square.png') 2x
    );
    content: image-set(
      TYPO('resources/blue-100-px-square.png') 1x,
      TYPO('resources/blue-100-px-square.png') 2x
    );
  }

</style>

<div>
  This test passes if the div below is a 100px red square.
</div>
<br>
<div id="foo"></div>