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diff --git a/testing/web-platform/tests/layout-instability/idlharness.html b/testing/web-platform/tests/layout-instability/idlharness.html
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+<!doctype html>
+<title>Layout Instability IDL tests</title>
+<meta name="timeout" content="long">
+<link rel="help" href="https://wicg.github.io/layout-instability/">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/resources/WebIDLParser.js"></script>
+<script src="/resources/idlharness.js"></script>
+<script src="resources/util.js"></script>
+<script>
+'use strict';
+
+idl_test(
+ ['layout-instability'],
+ ['performance-timeline', 'geometry', 'dom', 'hr-time'],
+ async (idl_array, t) => {
+ idl_array.add_objects({
+ LayoutShift: ['layoutShift'],
+ LayoutShiftAttribution: ['layoutShiftAttribution'],
+ });
+
+ // If LayoutShift isn't supported, avoid the timeout below and just let the
+ // objects declared above be null. The tests will still fail, but we will
+ // consistently generate the same set of subtests on all platforms.
+ if (!PerformanceObserver ||
+ !PerformanceObserver.supportedEntryTypes ||
+ !PerformanceObserver.supportedEntryTypes.includes('layout-shift')) {
+ return;
+ }
+
+ // Make sure that the image has initially been laid out, so that the movement
+ // later is counted as a layout shift.
+ await waitForAnimationFrames(2);
+
+ self.layoutShift = await new Promise((resolve, reject) => {
+ const observer = new PerformanceObserver(entryList => {
+ resolve(entryList.getEntries()[0]);
+ });
+ observer.observe({type: 'layout-shift', buffered: true});
+ t.step_timeout(() => reject('Timed out waiting for LayoutShift entry'), 3000);
+
+ // Move the image, to cause layout shift.
+ image.style.marginTop = '100px';
+ });
+ self.layoutShiftAttribution = layoutShift.sources[0];
+ }
+);
+</script>
+
+<img id="image" src="/images/green-100x50.png">