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diff --git a/testing/web-platform/tests/longtask-timing/longtask-attributes.html b/testing/web-platform/tests/longtask-timing/longtask-attributes.html
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+++ b/testing/web-platform/tests/longtask-timing/longtask-attributes.html
@@ -0,0 +1,48 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>LongTask Timing: validate long task attributes</title>
+<body>
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/utils.js"></script>
+
+<h1>Long Task Attributes</h1>
+<div id="log"></div>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceLongTaskTiming, 'Longtasks are not supported.');
+    const observer = new PerformanceObserver(
+        t.step_func(function (entryList) {
+            const entries = entryList.getEntries();
+            assert_equals(entries.length, 1,
+                'Exactly one entry is expected.');
+            const longtask = entries[0];
+            checkLongTaskEntry(longtask);
+            // Assert the TaskAttributionTiming entry in attribution.
+            assert_equals(longtask.attribution.length, 1,
+                'Exactly one attribution entry is expected');
+            const attribution = longtask.attribution[0];
+            assert_equals(attribution.entryType, 'taskattribution');
+            assert_equals(attribution.name, 'unknown');
+            assert_equals(attribution.duration, 0);
+            assert_equals(attribution.startTime, 0);
+            assert_equals(attribution.containerType, 'window');
+            assert_equals(attribution.containerId, '');
+            assert_equals(attribution.containerName, '');
+            assert_equals(attribution.containerSrc, '');
+            observer.disconnect();
+            t.done();
+        })
+    );
+    observer.observe({entryTypes: ['longtask']});
+
+    window.onload = () => {
+        /* Generate a slow task */
+        const begin = window.performance.now();
+        while (window.performance.now() < begin + 60);
+    };
+}, 'Performance longtask entries are observable.');
+</script>
+
+</body>