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diff --git a/testing/web-platform/tests/long-animation-frame/tentative/loaf-blocking-duration.html b/testing/web-platform/tests/long-animation-frame/tentative/loaf-blocking-duration.html
new file mode 100644
index 0000000000..e56a98649e
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+++ b/testing/web-platform/tests/long-animation-frame/tentative/loaf-blocking-duration.html
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+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Long Animation Frame Timing: basic</title>
+<meta name="timeout" content="long">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/utils.js"></script>
+
+<body>
+<h1>Long Animation Frame: blocking duration</h1>
+<div id="log"></div>
+<script>
+
+function loaf_blocking_duration_test(run, label) {
+  const OVERHEAD_EPSILON = 5;
+  const BLOCKING_THRESHOLD = 50;
+  promise_test(async t => {
+    let found = false;
+    for (let i = 0; i < 10 && !found; ++i) {
+      const longtask_promise = new Promise(resolve => new PerformanceObserver(
+      (entries, observer) => {
+        resolve(entries.getEntries());
+        observer.disconnect();
+      }).observe({entryTypes: ["longtask"]}));
+      const [longtask_entries, loaf_entry] = await Promise.all(
+        [longtask_promise, expect_long_frame(run, t)]);
+      const overlapping = longtask_entries.filter(longtask =>
+        (longtask.startTime >= loaf_entry.startTime &&
+          longtask.startTime < (loaf_entry.startTime + loaf_entry.duration) &&
+          (!loaf_entry.renderStart ||
+          (longtask.startTime < loaf_entry.renderStart - OVERHEAD_EPSILON))));
+
+      const longest_index = overlapping.reduce(
+        (max, cur, i) => cur > overlapping[max] ? i : max, 0);
+      let expected_blocking_duration = 0;
+      overlapping.forEach(({duration}, i) => {
+        if (i === longest_index && loaf_entry.renderStart)
+          duration += loaf_entry.startTime + loaf_entry.duration -
+          loaf_entry.renderStart;
+        expected_blocking_duration += Math.max(0, duration - BLOCKING_THRESHOLD);
+      });
+
+      if (!overlapping.length && loaf_entry.renderStart) {
+        expected_blocking_duration =
+          Math.max(0,
+            loaf_entry.startTime + loaf_entry.duration - loaf_entry.renderStart -
+            BLOCKING_THRESHOLD);
+      }
+
+      if (Math.abs(loaf_entry.blockingDuration - expected_blocking_duration) <
+          OVERHEAD_EPSILON) {
+        found = true;
+      }
+    }
+    assert_true(found);
+  }, `LoAF blockingDuration should be equivalent to long tasks: ${label}`);
+}
+
+loaf_blocking_duration_test(t => t.step_timeout(busy_wait), "Non-rendering");
+loaf_blocking_duration_test(t => t.step_timeout(() => {
+  busy_wait();
+  requestAnimationFrame(busy_wait);
+}), "Rendering");
+</script>
+</body>