background-image-relative-url-in-iframe.html

<!DOCTYPE html>
<iframe id="iframe"></iframe>
<script>
  if (window.testRunner) {
    testRunner.waitUntilDone();
  }

  var iframe = document.getElementById('iframe');
  iframe.srcdoc = `
    <style>
      .bg-img {
        background: url('./resources/green-24x24.png') no-repeat;
        width: 24px;
        height:24px;
      }
    </style>
    <div class='bg-img'></div>`;

  if (window.testRunner) {
    iframe.onload = function() { testRunner.notifyDone(); };
  }
</script>

chromium/third_party/blink/web_tests/fast/backgrounds/background-image-relative-url-in-iframe.html