progressively-loaded-image.html

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This WPT test may be referenced by the following Test IDs:
- /largest-contentful-paint/progressively-loaded-image.html - WPT Dashboard Interop Dashboard

<!DOCTYPE HTML>

<meta charset=utf-8>

<title>Largest Contentful Paint: Progressively Loaded Image</title>

<body>

  <script src="/resources/testharness.js"></script>

  <script src="/resources/testharnessreport.js"></script>

  <script src="resources/largest-contentful-paint-helpers.js"></script>

  <script src="/common/get-host-info.sub.js"></script>

  <script>

    promise_test(async function (t) {

      assert_implements(window.LargestContentfulPaint, "LargestContentfulPaint is not implemented");

      const beforeLoad = performance.now();

      const observer = new PerformanceObserver(

        t.step_func_done(function(entryList) {

          assert_equals(entryList.getEntries().length, 1);

          const entry = entryList.getEntries()[0];

          const imgElement = document.getElementById('image_id');

          const imageSrc =  imgElement.currentSrc;

          const imageWidth = imgElement.naturalWidth;

          const imageHeight = imgElement.naturalHeight;

          const size = imageWidth * imageHeight;

          const lcpLoadTime = entry.loadTime;

          const lcpRenderTime = entry.renderTime;

          const resLoadTime = calculateResourceLoadTime(imageSrc);

          assert_greater_than_equal(lcpLoadTime,resLoadTime,'Load before LCP event fired');

          assert_greater_than_equal(lcpRenderTime,resLoadTime,'Load before LCP event rendered');

          checkImage(entry, imageSrc, 'image_id', size, beforeLoad);

})

);

      observer.observe({type: 'largest-contentful-paint', buffered: true})

    }, 'Ensure Progressive JPEG LCP load and render events occur after resource loading');

    function calculateResourceLoadTime(imageSrc) {

      const entries = performance.getEntriesByName(imageSrc);

      const resourceEntry = entries[0];

      assert_equals(entries.length, 1, 'No Resource timing entry found');

      return resourceEntry.responseEnd;

  </script>

  <img src='/images/computer.jpg?pipe=trickle(982:d1)' id='image_id'/>

</body>

</html>