From 36d22d82aa202bb199967e9512281e9a53db42c9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 21:33:14 +0200
Subject: Adding upstream version 115.7.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 testing/web-platform/tests/element-timing/META.yml |   3 +
 .../element-timing/background-image-data-uri.html  |  39 +++++++
 .../background-image-multiple-elements.html        |  88 +++++++++++++++
 .../element-timing/background-image-stretched.html |  39 +++++++
 .../tests/element-timing/buffer-before-onload.html |  44 ++++++++
 .../tests/element-timing/buffered-flag.html        |  40 +++++++
 .../element-timing/cross-origin-element.sub.html   |  45 ++++++++
 .../cross-origin-iframe-element.sub.html           |  36 ++++++
 .../tests/element-timing/css-generated-text.html   |  36 ++++++
 .../tests/element-timing/disconnect-image.html     |  41 +++++++
 .../element-only-when-fully-active.html            |  20 ++++
 .../element-timing/first-letter-background.html    |  57 ++++++++++
 .../tests/element-timing/fixed-id-identifier.html  |  28 +++++
 .../tests/element-timing/idlharness.window.js      |  17 +++
 .../tests/element-timing/image-TAO.sub.html        |  54 +++++++++
 .../tests/element-timing/image-carousel.html       |  75 +++++++++++++
 .../tests/element-timing/image-clipped-svg.html    |  34 ++++++
 .../tests/element-timing/image-data-uri.html       |  37 +++++++
 .../tests/element-timing/image-not-added.html      |  31 ++++++
 .../element-timing/image-not-fully-visible.html    |  47 ++++++++
 .../tests/element-timing/image-rect-iframe.html    |  33 ++++++
 .../tests/element-timing/image-src-change.html     |  86 +++++++++++++++
 .../tests/element-timing/image-with-css-scale.html |  42 +++++++
 .../tests/element-timing/image-with-rotation.html  |  50 +++++++++
 .../element-timing/images-repeated-resource.html   |  73 ++++++++++++
 .../tests/element-timing/invisible-images.html     |  79 +++++++++++++
 .../element-timing/multiple-background-images.html |  58 ++++++++++
 .../element-timing/multiple-redirects-TAO.html     |  62 +++++++++++
 .../element-timing/observe-background-image.html   |  37 +++++++
 .../element-timing/observe-child-element.html      |  41 +++++++
 .../element-timing/observe-elementtiming.html      |  44 ++++++++
 .../element-timing/observe-empty-attribute.html    |  38 +++++++
 .../element-timing/observe-multiple-images.html    | 122 +++++++++++++++++++++
 .../tests/element-timing/observe-shadow-image.html |  38 +++++++
 .../tests/element-timing/observe-shadow-text.html  |  37 +++++++
 .../tests/element-timing/observe-svg-image.html    |  34 ++++++
 .../tests/element-timing/observe-text.html         |  48 ++++++++
 .../tests/element-timing/observe-video-poster.html |  32 ++++++
 .../element-timing/progressively-loaded-image.html |  40 +++++++
 .../tests/element-timing/rectangular-image.html    |  43 ++++++++
 .../tests/element-timing/redirects-tao-star.html   |  53 +++++++++
 .../tests/element-timing/resources/TAOImage.py     |  47 ++++++++
 .../tests/element-timing/resources/circle-tao.svg  |   6 +
 .../resources/circle-tao.svg.headers               |   1 +
 .../tests/element-timing/resources/circle.svg      |   6 +
 .../resources/element-timing-helpers.js            |  74 +++++++++++++
 .../resources/iframe-stores-entry.html             |  12 ++
 .../resources/iframe-with-content.html             |   5 +
 .../resources/iframe-with-square-sends-entry.html  |  26 +++++
 .../resources/iframe-with-square.html              |  10 ++
 .../element-timing/resources/multiple-redirects.py |  59 ++++++++++
 .../element-timing/resources/progressive-image.py  |  27 +++++
 .../tests/element-timing/resources/slow-image.py   |  19 ++++
 .../tests/element-timing/resources/square100.png   | Bin 0 -> 12940 bytes
 .../tests/element-timing/resources/square20.jpg    | Bin 0 -> 529 bytes
 .../tests/element-timing/resources/square20.png    | Bin 0 -> 150 bytes
 .../tests/element-timing/retrievability.html       |  29 +++++
 .../element-timing/same-origin-redirects.html      |  37 +++++++
 .../tests/element-timing/scroll-to-text.html       |  30 +++++
 .../element-timing/supported-element-type.html     |  17 +++
 .../element-timing/text-with-display-style.html    |  54 +++++++++
 .../web-platform/tests/element-timing/toJSON.html  |  45 ++++++++
 62 files changed, 2405 insertions(+)
 create mode 100644 testing/web-platform/tests/element-timing/META.yml
 create mode 100644 testing/web-platform/tests/element-timing/background-image-data-uri.html
 create mode 100644 testing/web-platform/tests/element-timing/background-image-multiple-elements.html
 create mode 100644 testing/web-platform/tests/element-timing/background-image-stretched.html
 create mode 100644 testing/web-platform/tests/element-timing/buffer-before-onload.html
 create mode 100644 testing/web-platform/tests/element-timing/buffered-flag.html
 create mode 100644 testing/web-platform/tests/element-timing/cross-origin-element.sub.html
 create mode 100644 testing/web-platform/tests/element-timing/cross-origin-iframe-element.sub.html
 create mode 100644 testing/web-platform/tests/element-timing/css-generated-text.html
 create mode 100644 testing/web-platform/tests/element-timing/disconnect-image.html
 create mode 100644 testing/web-platform/tests/element-timing/element-only-when-fully-active.html
 create mode 100644 testing/web-platform/tests/element-timing/first-letter-background.html
 create mode 100644 testing/web-platform/tests/element-timing/fixed-id-identifier.html
 create mode 100644 testing/web-platform/tests/element-timing/idlharness.window.js
 create mode 100644 testing/web-platform/tests/element-timing/image-TAO.sub.html
 create mode 100644 testing/web-platform/tests/element-timing/image-carousel.html
 create mode 100644 testing/web-platform/tests/element-timing/image-clipped-svg.html
 create mode 100644 testing/web-platform/tests/element-timing/image-data-uri.html
 create mode 100644 testing/web-platform/tests/element-timing/image-not-added.html
 create mode 100644 testing/web-platform/tests/element-timing/image-not-fully-visible.html
 create mode 100644 testing/web-platform/tests/element-timing/image-rect-iframe.html
 create mode 100644 testing/web-platform/tests/element-timing/image-src-change.html
 create mode 100644 testing/web-platform/tests/element-timing/image-with-css-scale.html
 create mode 100644 testing/web-platform/tests/element-timing/image-with-rotation.html
 create mode 100644 testing/web-platform/tests/element-timing/images-repeated-resource.html
 create mode 100644 testing/web-platform/tests/element-timing/invisible-images.html
 create mode 100644 testing/web-platform/tests/element-timing/multiple-background-images.html
 create mode 100644 testing/web-platform/tests/element-timing/multiple-redirects-TAO.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-background-image.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-child-element.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-elementtiming.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-empty-attribute.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-multiple-images.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-shadow-image.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-shadow-text.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-svg-image.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-text.html
 create mode 100644 testing/web-platform/tests/element-timing/observe-video-poster.html
 create mode 100644 testing/web-platform/tests/element-timing/progressively-loaded-image.html
 create mode 100644 testing/web-platform/tests/element-timing/rectangular-image.html
 create mode 100644 testing/web-platform/tests/element-timing/redirects-tao-star.html
 create mode 100644 testing/web-platform/tests/element-timing/resources/TAOImage.py
 create mode 100644 testing/web-platform/tests/element-timing/resources/circle-tao.svg
 create mode 100644 testing/web-platform/tests/element-timing/resources/circle-tao.svg.headers
 create mode 100644 testing/web-platform/tests/element-timing/resources/circle.svg
 create mode 100644 testing/web-platform/tests/element-timing/resources/element-timing-helpers.js
 create mode 100644 testing/web-platform/tests/element-timing/resources/iframe-stores-entry.html
 create mode 100644 testing/web-platform/tests/element-timing/resources/iframe-with-content.html
 create mode 100644 testing/web-platform/tests/element-timing/resources/iframe-with-square-sends-entry.html
 create mode 100644 testing/web-platform/tests/element-timing/resources/iframe-with-square.html
 create mode 100644 testing/web-platform/tests/element-timing/resources/multiple-redirects.py
 create mode 100644 testing/web-platform/tests/element-timing/resources/progressive-image.py
 create mode 100644 testing/web-platform/tests/element-timing/resources/slow-image.py
 create mode 100644 testing/web-platform/tests/element-timing/resources/square100.png
 create mode 100644 testing/web-platform/tests/element-timing/resources/square20.jpg
 create mode 100644 testing/web-platform/tests/element-timing/resources/square20.png
 create mode 100644 testing/web-platform/tests/element-timing/retrievability.html
 create mode 100644 testing/web-platform/tests/element-timing/same-origin-redirects.html
 create mode 100644 testing/web-platform/tests/element-timing/scroll-to-text.html
 create mode 100644 testing/web-platform/tests/element-timing/supported-element-type.html
 create mode 100644 testing/web-platform/tests/element-timing/text-with-display-style.html
 create mode 100644 testing/web-platform/tests/element-timing/toJSON.html

(limited to 'testing/web-platform/tests/element-timing')

diff --git a/testing/web-platform/tests/element-timing/META.yml b/testing/web-platform/tests/element-timing/META.yml
new file mode 100644
index 0000000000..15510df7e5
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/META.yml
@@ -0,0 +1,3 @@
+spec: https://wicg.github.io/element-timing/
+suggested_reviewers:
+  - npm1
diff --git a/testing/web-platform/tests/element-timing/background-image-data-uri.html b/testing/web-platform/tests/element-timing/background-image-data-uri.html
new file mode 100644
index 0000000000..9722463742
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/background-image-data-uri.html
@@ -0,0 +1,39 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe element with inline background image</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#target {
+  width: 100px;
+  height: 50px;
+  background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKAQMAAAC3/F3+AAAABGdBTUEAALGPC/xhBQAAACBjSFJNAAB6JgAAgIQAAPoAAACA6AAAdTAAAOpgAAA6mAAAF3CculE8AAAABlBMVEX/AAD///9BHTQRAAAAAWJLR0QB/wIt3gAAAAtJREFUCNdjYMAHAAAeAAFuhUcyAAAAJXRFWHRkYXRlOmNyZWF0ZQAyMDE5LTA0LTA1VDIwOjA4OjQxKzAyOjAwPa6EZwAAACV0RVh0ZGF0ZTptb2RpZnkAMjAxOS0wNC0wNVQyMDowODo0MSswMjowMEzzPNsAAAAASUVORK5CYII=);
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        // Only the first characters of the data URI are included in the entry.
+        const uriPrefix = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKAQMAAAC3/F3+AAAABGdBTUEAALGPC/xhBQAAACBjSFJNAA';
+        checkElementWithoutResourceTiming(entry, uriPrefix, 'my_div', 'target',
+            beforeRender, document.getElementById('target'));
+        // The background image is a red square of length 10.
+        checkRect(entry, [0, 100, 0, 50]);
+        checkNaturalSize(entry, 10, 10);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Element with elementtiming attribute and inline background image is observable.');
+</script>
+<div id='target' elementtiming='my_div'></div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/background-image-multiple-elements.html b/testing/web-platform/tests/element-timing/background-image-multiple-elements.html
new file mode 100644
index 0000000000..11f8c05f96
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/background-image-multiple-elements.html
@@ -0,0 +1,88 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: background image affecting multiple elements</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+.my_div {
+  background-image: url('resources/square100.png');
+}
+#div1 {
+  width: 100px;
+  height: 100px;
+}
+#div2 {
+  width: 200px;
+  height: 100px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let beforeRender = performance.now();
+    let numObservedElements = 0;
+    let observedDiv1 = false;
+    let observedDiv2Img = false;
+    let observedDiv2Txt = false;
+    const pathname = window.location.origin + '/element-timing/resources/square100.png';
+    const observer = new PerformanceObserver(
+      t.step_func(function(entryList) {
+        entryList.getEntries().forEach(entry => {
+          numObservedElements++;
+          if (entry.id == 'div1') {
+            observedDiv1 = true;
+            checkElement(entry, pathname, 'et1', 'div1', beforeRender,
+                document.getElementById('div1'));
+            // Div is in the top left corner.
+            checkRect(entry, [0, 100, 0, 100]);
+            checkNaturalSize(entry, 100, 100);
+          }
+          else if (entry.id == 'div2') {
+            // Check image entry.
+            if (entry.name !== 'text-paint') {
+              observedDiv2Img = true;
+              checkElement(entry, pathname, 'et2', 'div2', beforeRender,
+                  document.getElementById('div2'));
+              // Div is below div1, on the left.
+              checkRect(entry, [0, 200, 100, 200]);
+              checkNaturalSize(entry, 100, 100);
+            }
+            // Check the text entry.
+            else {
+              observedDiv2Txt = true;
+              checkTextElement(entry, 'et2', 'div2', beforeRender,
+                document.getElementById('div2'));
+              assert_greater_than_equal(entry.intersectionRect.right - entry.intersectionRect.left, 50);
+              assert_greater_than_equal(entry.intersectionRect.bottom - entry.intersectionRect.top, 10);
+            }
+          }
+          else {
+            assert_unreached("Should not observe other elements!");
+          }
+          if (numObservedElements === 3) {
+            assert_true(observedDiv1);
+            assert_true(observedDiv2Img);
+            assert_true(observedDiv2Txt);
+            t.done();
+          }
+        });
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Background image affecting various elements is observed.');
+</script>
+<div id="div1" class="my_div" elementtiming="et1">
+  <img width=50 height=50 src='resources/circle.svg'/>
+</div>
+<div width=200 height=100 id="div2" class="my_div" elementtiming="et2">
+  Sample text inside div.
+</div>
+<div id="div3"/>
+  I am a div that should not be observed!
+</div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/background-image-stretched.html b/testing/web-platform/tests/element-timing/background-image-stretched.html
new file mode 100644
index 0000000000..3ad8976ccf
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/background-image-stretched.html
@@ -0,0 +1,39 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe larger element with stretched background image</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#target {
+  width: 200px;
+  height: 150px;
+  background-image: url('resources/square100.png');
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/resources/square100.png';
+        checkElement(entry, pathname, 'my_div', 'target', beforeRender,
+            document.getElementById('target'));
+        // The background image extends to occupy to full size of the div.
+        checkRect(entry, [0, 200, 0, 150]);
+        // The natural size of the square remains unchanged.
+        checkNaturalSize(entry, 100, 100);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Element with background image shows correct image size.');
+</script>
+<div id='target' elementtiming='my_div'></div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/buffer-before-onload.html b/testing/web-platform/tests/element-timing/buffer-before-onload.html
new file mode 100644
index 0000000000..17c8238f6e
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/buffer-before-onload.html
@@ -0,0 +1,44 @@
+<!DOCTYPE html>
+<html>
+<meta charset=utf-8 />
+<title>Element Timing: buffer elements before onload</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script src="resources/element-timing-helpers.js"></script>
+<body>
+<img src=resources/slow-image.py?name=square20.png&sleep=500>
+<script>
+  /*
+  In this test, a slow image is added to the frame to delay onload. The entry
+  is available from the observer with the buffered flag set to true.
+  */
+  async_test(function(t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    beforeRender = performance.now();
+    const img = document.createElement('img');
+    img.src = 'resources/square20.jpg';
+    img.setAttribute('elementtiming', 'my_image');
+    img.setAttribute('id', 'my_id');
+    document.body.appendChild(img);
+
+    // this PerformanceObserver should be notified about the previously
+    // buffered element entry
+    new PerformanceObserver(t.step_func((entryList, observer) => {
+      assert_equals(entryList.getEntries().length, 1);
+      entryList.getEntries().forEach(entry => {
+        assert_equals(entry.entryType, "element");
+        const pathname = window.location.origin + '/element-timing/resources/square20.jpg';
+        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);
+        checkNaturalSize(entry, 20, 20);
+        observer.disconnect();
+        t.done();
+      });
+    })).observe({
+      type: "element",
+      buffered: true
+    });
+  }, "Element Timing: image loads before onload available from buffered flag.");
+
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/element-timing/buffered-flag.html b/testing/web-platform/tests/element-timing/buffered-flag.html
new file mode 100644
index 0000000000..e7fcf059fb
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/buffered-flag.html
@@ -0,0 +1,40 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe element with buffered flag</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test(t => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const beforeRender = performance.now();
+    const img = document.createElement('img');
+    // Initial observer used to know when entry has been dispatched
+    new PerformanceObserver(() => {
+      // Second observer should require buffered flag to receive the already-dispatched entry.
+      new PerformanceObserver(t.step_func_done(entryList => {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/images/black-rectangle.png';
+        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, document.getElementById('my_id'));
+        // Test that viewport size is at least 100, so element is fully visible.
+        assert_greater_than_equal(document.documentElement.clientWidth, 100, 'Width should be >= 100');
+        assert_greater_than_equal(document.documentElement.clientHeight, 100, 'Height should be >= 100');
+
+        checkRect(entry, [0, 100, 0, 50]);
+        checkNaturalSize(entry, 100, 50);
+      })).observe({type: 'element', buffered: true});
+    }).observe({type: 'element'});
+    img.src = '/images/black-rectangle.png';
+    img.setAttribute('elementtiming', 'my_image');
+    img.setAttribute('id', 'my_id');
+    document.body.appendChild(img);
+  }, 'Element Timing entries are observable via buffered flag.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/cross-origin-element.sub.html b/testing/web-platform/tests/element-timing/cross-origin-element.sub.html
new file mode 100644
index 0000000000..1052bf246d
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/cross-origin-element.sub.html
@@ -0,0 +1,45 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe cross-origin images but without renderTime</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let img;
+    const pathname = 'http://{{domains[www]}}:{{ports[http][1]}}'
+          + '/element-timing/resources/square100.png';
+    const observer = new PerformanceObserver(
+      t.step_func_done((entryList) => {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        checkElement(entry, pathname, 'my_image', 'the_id', 0, img);
+        assert_equals(entry.renderTime, 0,
+          'The renderTime of a cross-origin image should be 0.');
+        checkRect(entry, [0, 100, 0, 100]);
+        checkNaturalSize(entry, 100, 100);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    // TODO(npm): change observer to use buffered flag.
+    window.onload = t.step_func(() => {
+      // Add a cross origin image resource.
+      img = document.createElement('img');
+      img.src = pathname;
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'the_id');
+      document.body.appendChild(img);
+    });
+  }, 'Cross-origin image element is NOT observable.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/cross-origin-iframe-element.sub.html b/testing/web-platform/tests/element-timing/cross-origin-iframe-element.sub.html
new file mode 100644
index 0000000000..2f49933ab9
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/cross-origin-iframe-element.sub.html
@@ -0,0 +1,36 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: do NOT observe elements from cross-origin iframes</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done((entryList) => {
+        assert_unreached("We should not observe a cross origin element.");
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the iframe during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    // TODO(npm): change observer to use buffered flag.
+    window.onload = t.step_func(() => {
+      // Add a cross origin iframe with an image.
+      const iframe = document.createElement('iframe');
+      iframe.src = 'http://{{domains[www]}}:{{ports[http][1]}}'
+          + '/element-timing/resources/iframe-with-square.html';
+      document.body.appendChild(iframe);
+      iframe.onload = t.step_func(() => {
+        t.step_timeout( () => {
+          // After some wait, assume observer did not receive the entry, so the test passes.
+          t.done();
+        }, 100);
+      });
+    });
+  }, 'Element from cross origin iframe is NOT observable.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/css-generated-text.html b/testing/web-platform/tests/element-timing/css-generated-text.html
new file mode 100644
index 0000000000..d1bbf5a7ee
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/css-generated-text.html
@@ -0,0 +1,36 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe generated CSS text</title>
+<style>
+p::before {
+  content: "Generated text prefix";
+}
+body {
+  margin: 20px;
+}
+</style>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<p elementtiming='my_text' id='text_id'></p>
+<script>
+  async_test(function (t) {
+    const beforeRender = performance.now();
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        checkTextElement(entry, 'my_text', 'text_id', beforeRender, document.getElementById('text_id'));
+        assert_equals(entry.intersectionRect.left, 20, 'left should be 20.');
+        assert_equals(entry.intersectionRect.top, 20, 'top should be 20.');
+        // Try a lower bound of height=10, width=80 for the generated text.
+        assert_greater_than_equal(entry.intersectionRect.right, 100);
+        assert_greater_than_equal(entry.intersectionRect.bottom, 30);
+      })
+    );
+    observer.observe({type: 'element', buffered: true});
+  }, 'Generated text content is observable.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/disconnect-image.html b/testing/web-platform/tests/element-timing/disconnect-image.html
new file mode 100644
index 0000000000..6f9f2ce7b9
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/disconnect-image.html
@@ -0,0 +1,41 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: element attribute returns null when element is disconnected</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender;
+  let img;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/resources/square100.png';
+        // This method will check that entry.element is |img|.
+        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);
+
+        img.parentNode.removeChild(img);
+        // After removing image, entry.element should return null.
+        assert_equals(entry.element, null);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width equal to 100 and height equal to 100.
+      img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'my_id');
+      document.body.appendChild(img);
+      beforeRender = performance.now();
+    };
+  }, 'Disconnected elements have null as their |element| attribute.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/element-only-when-fully-active.html b/testing/web-platform/tests/element-timing/element-only-when-fully-active.html
new file mode 100644
index 0000000000..ff08074d57
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/element-only-when-fully-active.html
@@ -0,0 +1,20 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>ElementTiming: element is only exposed for fully active documents.</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<iframe src="resources/iframe-stores-entry.html" id="ifr"></iframe>
+<script>
+  let t = async_test('Only expose element attribute for fully active documents');
+  t.step(() => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+  });
+  window.triggerTest = t.step_func_done(elementEntry => {
+    assert_not_equals(elementEntry.element, null);
+    const iframe = document.getElementById('ifr');
+    iframe.remove();
+    assert_equals(elementEntry.element, null);
+  });
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/first-letter-background.html b/testing/web-platform/tests/element-timing/first-letter-background.html
new file mode 100644
index 0000000000..f05f2f9223
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/first-letter-background.html
@@ -0,0 +1,57 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe element with background image in its first letter</title>
+<body>
+<style>
+#target::first-letter {
+  background-image: url('/images/black-rectangle.png');
+}
+#target {
+  font-size: 12px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const div = document.getElementById('target');
+    let textObserved = false;
+    let imageObserved = false;
+    function calculateSize(entry) {
+      const ir = entry.intersectionRect;
+      return (ir.right - ir.left) * (ir.bottom - ir.top);
+    }
+    const observer = new PerformanceObserver(
+      t.step_func(function(entryList) {
+        entryList.getEntries().forEach(entry => {
+          if (entry.name === 'text-paint') {
+            checkTextElement(entry, 'my_div', 'target', beforeRender, div);
+            textObserved = true;
+            const size = calculateSize(entry);
+            // Assume average width is between 4px and 15px.
+            // Therefore, text size must be between 12 * (35*4) and 12 * (35*15).
+            assert_between_inclusive(size, 1680, 6300);
+          }
+          else {
+            assert_equals(entry.name, 'image-paint');
+            const pathname = window.location.origin + '/images/black-rectangle.png';
+            checkElement(entry, pathname, 'my_div', 'target', beforeRender, div);
+            checkNaturalSize(entry, 100, 50);
+            imageObserved = true;
+            const size = calculateSize(entry);
+            // Background image size should only be approximately the size of a single letter.
+            assert_between_inclusive(size, 48, 180);
+          }
+        });
+        if (textObserved && imageObserved)
+          t.done();
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Element with elementtiming attribute and background image in first-letter is observable.');
+</script>
+<div id='target' elementtiming='my_div'>This is some text that I care about</div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/fixed-id-identifier.html b/testing/web-platform/tests/element-timing/fixed-id-identifier.html
new file mode 100644
index 0000000000..749b9ada2f
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/fixed-id-identifier.html
@@ -0,0 +1,28 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: entry does not change its id or identifier value</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<p elementtiming='my_identifier' id='my_id'>Text</p>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        assert_equals(entry.id, 'my_id');
+        assert_equals(entry.identifier, 'my_identifier');
+        const element = document.getElementById('my_id');
+        element.id = 'other_id';
+        element.setAttribute('elementtiming', 'other_identifier');
+        assert_equals(entry.id, 'my_id');
+        assert_equals(entry.identifier, 'my_identifier');
+      })
+    );
+    observer.observe({type: 'element', buffered: true});
+  }, 'PerformanceElementTiming id and identifier do not change when Element changes.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/idlharness.window.js b/testing/web-platform/tests/element-timing/idlharness.window.js
new file mode 100644
index 0000000000..a3189c3679
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/idlharness.window.js
@@ -0,0 +1,17 @@
+// META: script=/resources/WebIDLParser.js
+// META: script=/resources/idlharness.js
+// META: timeout=long
+
+// https://wicg.github.io/element-timing/
+
+'use strict';
+
+idl_test(
+  ['element-timing'],
+  ['performance-timeline', 'dom'],
+  idl_array => {
+    idl_array.add_objects({
+      // PerformanceElementTiming: [ TODO ]
+    });
+  }
+);
diff --git a/testing/web-platform/tests/element-timing/image-TAO.sub.html b/testing/web-platform/tests/element-timing/image-TAO.sub.html
new file mode 100644
index 0000000000..7b455dfa4c
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-TAO.sub.html
@@ -0,0 +1,54 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe cross origin images with various Timing-Allow-Origin headers</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test(t => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const beforeRender = performance.now();
+    const remote_img = 'http://{{domains[www]}}:{{ports[http][1]}}/element-timing/resources/TAOImage.py?'
+        + 'origin=' + window.location.origin +'&tao=';
+    const valid_tao = ['wildcard', 'origin', 'multi', 'multi_wildcard', 'match_origin', 'match_wildcard'];
+    function addImage(tao) {
+      const img = document.createElement('img');
+      img.src = remote_img + tao;
+      img.setAttribute('elementtiming', tao);
+      img.id = 'id_' + tao;
+      document.body.appendChild(img);
+    }
+    valid_tao.forEach(tao => {
+      addImage(tao);
+    });
+    const invalid_tao = ['null', 'space', 'uppercase'];
+    invalid_tao.forEach(tao => {
+      addImage(tao);
+    });
+    let img_count = 0;
+    const total_images = valid_tao.length + invalid_tao.length;
+    new PerformanceObserver(
+      t.step_func(entryList => {
+        entryList.getEntries().forEach(entry => {
+          img_count++;
+          const tao = entry.identifier;
+          const img = document.getElementById('id_' + tao);
+          if (valid_tao.includes(tao)) {
+            checkElement(entry, remote_img + tao, tao, 'id_' + tao, beforeRender, img);
+          } else if (invalid_tao.includes(tao)) {
+            assert_equals(entry.renderTime, 0, 'Entry with tao=' + tao + ' must have a renderTime of 0');
+            checkElement(entry, remote_img + tao, tao, 'id_' + tao, 0, img);
+          }
+          else {
+            assert_unreached('Should be in one of valid_tao OR invalid_tao');
+          }
+          checkNaturalSize(entry, 100, 100);
+          if (img_count == total_images)
+            t.done();
+        });
+      })
+    ).observe({type: 'element', buffered: true});
+  }, 'Cross-origin elements with valid TAO have correct renderTime, with invalid TAO have renderTime set to 0.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-carousel.html b/testing/web-platform/tests/element-timing/image-carousel.html
new file mode 100644
index 0000000000..2b3b618f8c
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-carousel.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Element Timing: observe images in carousel</title>
+<style>
+body {
+  margin: 0;
+}
+/* Do not display images by default */
+.carousel-image {
+  display: none;
+}
+</style>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+
+<div class="slideshow-container">
+  <div class='carousel-image'>
+    <img src="resources/circle.svg" elementtiming='image0' id='image0'>
+  </div>
+  <div class='carousel-image'>
+    <img src="resources/square100.png" elementtiming='image1' id='image1'>
+  </div>
+</div>
+
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const beforeRenderTimes = [];
+    let entry_count = 0;
+    const entry_count_per_element = [0, 0];
+    const pathname0 = window.location.origin + '/element-timing/resources/circle.svg';
+    const pathname1 = window.location.origin + '/element-timing/resources/square100.png';
+    const observer = new PerformanceObserver(t.step_func(list => {
+      list.getEntries().forEach(entry => {
+        if (entry_count % 2 == 0) {
+          checkElement(entry, pathname0, 'image0', 'image0', beforeRenderTimes[entry_count],
+              document.getElementById('image0'));
+          checkRect(entry, [0, 200, 0, 200]);
+          checkNaturalSize(entry, 200, 200);
+          entry_count_per_element[0]++;
+        }
+        else {
+          checkElement(entry, pathname1, 'image1', 'image1', beforeRenderTimes[entry_count],
+              document.getElementById('image1'));
+          checkRect(entry, [0, 100, 0, 100]);
+          checkNaturalSize(entry, 100, 100);
+          entry_count_per_element[1]++;
+        }
+        entry_count++;
+        // Check each image twice before ending the test.
+        if (entry_count == 4) {
+          assert_equals(entry_count_per_element[0], 2);
+          assert_equals(entry_count_per_element[1], 2);
+          t.done();
+        }
+      })
+    }));
+    observer.observe({entryTypes: ['element']});
+    let slideIndex = 0;
+    showCarousel();
+
+    function showCarousel() {
+      beforeRenderTimes.push(performance.now());
+      const slides = document.getElementsByClassName("carousel-image");
+      slides[slideIndex].style.display = "block";
+      slides[1 - slideIndex].style.display = "none";
+      slideIndex = 1 - slideIndex;
+      t.step_timeout(showCarousel, 50); // Change image every 50 ms.
+    }
+  }, 'Entries for elements within an image carousel are dispatched when the elements are redrawn.');
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/element-timing/image-clipped-svg.html b/testing/web-platform/tests/element-timing/image-clipped-svg.html
new file mode 100644
index 0000000000..4c2bb36079
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-clipped-svg.html
@@ -0,0 +1,34 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe image inside SVG with small dimensions</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+let beforeRender;
+async_test(function (t) {
+  assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+  const observer = new PerformanceObserver(
+    t.step_func_done(function(entryList) {
+      assert_equals(entryList.getEntries().length, 1);
+      const entry = entryList.getEntries()[0];
+      const pathname = window.location.origin + '/element-timing/resources/circle.svg';
+      checkElement(entry, pathname, 'my_svg', 'SVG', beforeRender,
+          document.getElementById('SVG'));
+      // Image size is 200x200 but SVG size is 100x100 so it is clipped.
+      checkRect(entry, [0, 100, 0, 100]);
+      checkNaturalSize(entry, 200, 200);
+    })
+  );
+  observer.observe({entryTypes: ['element']});
+  beforeRender = performance.now();
+}, "Able to observe svg image.");
+</script>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<svg width="100" height="100">
+  <image href='resources/circle.svg' elementtiming='my_svg' id='SVG'/>
+</svg>
diff --git a/testing/web-platform/tests/element-timing/image-data-uri.html b/testing/web-platform/tests/element-timing/image-data-uri.html
new file mode 100644
index 0000000000..02d88fb244
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-data-uri.html
@@ -0,0 +1,37 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe inline image</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#inline_wee {
+  display: block;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        // Only the first characters of the data URI are included in the entry.
+        const uriPrefix = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKAQMAAAC3/F3+AAAABGdBTUEAALGPC/xhBQAAACBjSFJNAA';
+        checkElementWithoutResourceTiming(entry, uriPrefix, 'my_img', 'inline_wee',
+            beforeRender, document.getElementById('inline_wee'));
+        // The image is a red square of length 10.
+        checkRect(entry, [0, 10, 0, 10]);
+        checkNaturalSize(entry, 10, 10);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Inline image is observable via Element Timing.');
+</script>
+<img elementtiming='my_img' id='inline_wee' src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKAQMAAAC3/F3+AAAABGdBTUEAALGPC/xhBQAAACBjSFJNAAB6JgAAgIQAAPoAAACA6AAAdTAAAOpgAAA6mAAAF3CculE8AAAABlBMVEX/AAD///9BHTQRAAAAAWJLR0QB/wIt3gAAAAtJREFUCNdjYMAHAAAeAAFuhUcyAAAAJXRFWHRkYXRlOmNyZWF0ZQAyMDE5LTA0LTA1VDIwOjA4OjQxKzAyOjAwPa6EZwAAACV0RVh0ZGF0ZTptb2RpZnkAMjAxOS0wNC0wNVQyMDowODo0MSswMjowMEzzPNsAAAAASUVORK5CYII="/>
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-not-added.html b/testing/web-platform/tests/element-timing/image-not-added.html
new file mode 100644
index 0000000000..d77049ecd4
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-not-added.html
@@ -0,0 +1,31 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: do not observe a disconnected image</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(() => {
+        // The image should not have caused an entry, so fail test.
+        assert_unreached('Should not have received an entry!');
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width equal to 100 and height equal to 100.
+      const img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'my_id');
+      // Image has been created but not added.
+      // Wait for 500ms and end test, ensuring no entry was created.
+      t.step_timeout(() => {
+        t.done();
+      }, 500);
+    };
+  }, 'Image which is not added to DOM tree is not observable.');
+</script>
diff --git a/testing/web-platform/tests/element-timing/image-not-fully-visible.html b/testing/web-platform/tests/element-timing/image-not-fully-visible.html
new file mode 100644
index 0000000000..504d17592f
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-not-fully-visible.html
@@ -0,0 +1,47 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: intersectionRect when image overflows</title>
+<body>
+<style>
+body {
+  margin: 200px 100px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender;
+  let img;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/resources/square20.png';
+        checkElement(entry, pathname, 'not_fully_visible', '', beforeRender, img);
+        // Image will not be fully visible. It should start from the top left part
+        // of the document, excluding the margin, and then overflow.
+        checkRect(entry,
+          [100, document.documentElement.clientWidth, 200, document.documentElement.clientHeight]);
+        checkNaturalSize(entry, 20, 20);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add an image setting width and height equal to viewport.
+      img = document.createElement('img');
+      img.src = 'resources/square20.png';
+      img.setAttribute('elementtiming', 'not_fully_visible');
+      img.width = document.documentElement.clientWidth;
+      img.height = document.documentElement.clientHeight;
+      document.body.appendChild(img);
+      beforeRender = performance.now();
+    };
+  }, 'The intersectionRect of an img element overflowing is computed correctly');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-rect-iframe.html b/testing/web-platform/tests/element-timing/image-rect-iframe.html
new file mode 100644
index 0000000000..00986366e6
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-rect-iframe.html
@@ -0,0 +1,33 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: check intersectionRect for element in iframe</title>
+<body>
+<style>
+body {
+  margin: 50px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    on_event(window, 'message', e => {
+      assert_equals(e.data.length, 1);
+      assert_equals(e.data.entryType, 'element');
+      const rect = e.data.rect;
+      // rect should start at (0,0) even though main frame has a margin.
+      assert_equals(rect.left, 0);
+      assert_equals(rect.right, 100);
+      assert_equals(rect.top, 0);
+      assert_equals(rect.bottom, 100);
+      assert_equals(e.data.naturalWidth, 100);
+      assert_equals(e.data.naturalHeight, 100);
+      assert_equals(e.data.id, 'iframe_img_id');
+      assert_equals(e.data.elementId, 'iframe_img_id');
+      t.done();
+    });
+  }, 'Element Timing entry in iframe has coordinates relative to the iframe.');
+</script>
+<iframe src="resources/iframe-with-square-sends-entry.html"/>
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-src-change.html b/testing/web-platform/tests/element-timing/image-src-change.html
new file mode 100644
index 0000000000..7416a3f365
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-src-change.html
@@ -0,0 +1,86 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: src change triggers new entry</title>
+
+<body>
+  <style>
+    body {
+      margin: 0;
+    }
+
+  </style>
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+  <script src="resources/element-timing-helpers.js"></script>
+  <img elementtiming='my_image' id='my_id' />
+  <script>
+    setup({"hide_test_state": true});
+
+    const performanceEntryPromise = (pathname) => {
+      return new Promise(resolve => {
+        new PerformanceObserver((entryList, observer) => {
+          assert_equals(entryList.getEntries().length, 1);
+          if (entryList.getEntries()[0].url == pathname) {
+            observer.disconnect();
+            resolve(entryList.getEntries()[0]);
+          }
+        }).observe({ type: 'element' });
+      });
+    }
+
+    promise_test(async (t) => {
+      assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+
+      // Take beforeRender timestamp.
+      const beforeRender1 = performance.now();
+
+      const img = document.getElementById('my_id');
+
+      const url1 = 'resources/square100.png';
+
+      const pathname1 = (new URL(url1, document.location)).href
+
+      // Register performance observer.
+      const promise1 = performanceEntryPromise(pathname1);
+
+      //Load image
+      await new Promise(resolve => {
+        img.addEventListener('load', resolve);
+        img.src = url1;
+      });
+
+      // Get element entry.
+      const entry1 = await promise1;
+
+      // Check entry.
+      checkElement(entry1, pathname1, 'my_image', 'my_id', beforeRender1, img);
+      checkRect(entry1, [0, 100, 0, 100]);
+      checkNaturalSize(entry1, 100, 100);
+
+      // Take beforeRender timestamp before changing image src.
+      const beforeRender2 = performance.now();
+
+      // Set the src to trigger another entry.
+      const url2 = '/images/black-rectangle.png';
+
+      const pathname2 = (new URL(url2, document.location)).href;
+
+      const promise2 = performanceEntryPromise(pathname2);
+
+      //Load image with changed src.
+      await new Promise(resolve => {
+        img.addEventListener('load', resolve);
+        img.src = url2;
+      });
+
+      // Get the corresponding element entry.
+      const entry2 = await promise2;
+
+      // Check entry.
+      checkElement(entry2, pathname2, 'my_image', 'my_id', beforeRender2, img);
+      checkRect(entry2, [0, 100, 0, 50]);
+      checkNaturalSize(entry2, 100, 50);
+    }, 'Element Timing: changing src causes a new entry to be dispatched.')
+  </script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-with-css-scale.html b/testing/web-platform/tests/element-timing/image-with-css-scale.html
new file mode 100644
index 0000000000..a0490f375c
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-with-css-scale.html
@@ -0,0 +1,42 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: image with scaling.</title>
+<head>
+<style>
+/* The margin of 50px allows the rect to be fully shown when the div is scaled. */
+body {
+  margin: 50px;
+}
+.my_div {
+  width: 100px;
+  height: 50px;
+  transform: scale(2);
+}
+</style>
+</head>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  const beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/images/black-rectangle.png';
+        checkElement(entry, pathname, 'rectangle', 'rect_id', beforeRender,
+            document.getElementById('rect_id'));
+        checkRect(entry, [0, 200, 25, 125]);
+        checkNaturalSize(entry, 100, 50);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Image intersectionRect is affected by scaling, but not its intrinsic size.');
+</script>
+<div class="my_div" id='div_id'>
+  <img src="/images/black-rectangle.png" elementtiming="rectangle" id='rect_id'/>
+</div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/image-with-rotation.html b/testing/web-platform/tests/element-timing/image-with-rotation.html
new file mode 100644
index 0000000000..229e9ae96c
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/image-with-rotation.html
@@ -0,0 +1,50 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: image with rotation.</title>
+<head>
+<style>
+body {
+  margin: 0px;
+}
+.my_div {
+  width: 100px;
+  height: 50px;
+  transform: rotate(45deg);
+  transform-origin: top left;
+}
+</style>
+</head>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  const beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/images/black-rectangle.png';
+        checkElement(entry, pathname, 'rectangle', 'rect_id', beforeRender,
+            document.getElementById('rect_id'));
+        checkNaturalSize(entry, 100, 50);
+        const rect = entry.intersectionRect;
+        // The div rotates with respect to the origin, so part of it will be invisible.
+        // The width of the visible part will be 100 / sqrt(2) and the height will be
+        // 100 / sqrt(2) + 50 / sqrt(2).
+        assert_equals(rect.left, 0);
+        // Checking precision only to the nearest integer.
+        assert_equals(Math.round(rect.right), 71);
+        assert_equals(rect.top, 0);
+        assert_equals(Math.round(rect.bottom), 106);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Image intersectionRect is affected by rotation, but not its intrinsic size.');
+</script>
+<div class="my_div" id="div_id">
+  <img src="/images/black-rectangle.png" elementtiming="rectangle" id="rect_id"/>
+</div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/images-repeated-resource.html b/testing/web-platform/tests/element-timing/images-repeated-resource.html
new file mode 100644
index 0000000000..f7296e05e7
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/images-repeated-resource.html
@@ -0,0 +1,73 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe elements with the same resource</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender;
+  let numEntries = 0;
+  let loadTime1;
+  let loadTime2;
+  let renderTime1;
+  let renderTime2;
+  let img;
+  let img2;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/resources/square100.png';
+        // Easier to check the |element| attribute here since element ID is the same for both images.
+        checkElement(entry, pathname, entry.identifier, 'image_id', beforeRender, null);
+        checkNaturalSize(entry, 100, 100);
+        if (entry.identifier === 'my_image') {
+          ++numEntries;
+          loadTime1 = entry.loadTime;
+          renderTime1 = entry.renderTime;
+          assert_equals(entry.element, img);
+
+          img2 = document.createElement('img');
+          img2.src = 'resources/square100.png';
+          img2.setAttribute('elementtiming', 'my_image2');
+          img2.setAttribute('id', 'image_id');
+          document.body.appendChild(img2);
+          beforeRender = performance.now();
+        }
+        else if (entry.identifier === 'my_image2') {
+          ++numEntries;
+          loadTime2 = entry.loadTime;
+          renderTime2 = entry.renderTime;
+          assert_equals(entry.element, img2);
+        }
+        if (numEntries == 2) {
+          assert_greater_than(loadTime2, loadTime1, 'Second image loads after first.');
+          assert_greater_than(renderTime2, renderTime1, 'Second image renders after first');
+          t.done();
+        }
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the images during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width and height equal to 100.
+      img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'image_id');
+      document.body.appendChild(img);
+      beforeRender = performance.now();
+    };
+  }, 'Elements with elementtiming and same src are observable.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/invisible-images.html b/testing/web-platform/tests/element-timing/invisible-images.html
new file mode 100644
index 0000000000..ffde3ce2f6
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/invisible-images.html
@@ -0,0 +1,79 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: invisible images are not observable</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<style>
+  #opacity0 {
+    opacity: 0;
+  }
+  #visibilityHidden {
+    visibility: hidden;
+  }
+  #displayNone {
+    display: none;
+  }
+</style>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done((entries) => {
+        // The image should not have caused an entry, so fail test.
+        assert_unreached('Should not have received an entry! Received one with identifier '
+            + entries.getEntries()[0].identifier);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the images during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      const img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'opacity0');
+      document.body.appendChild(img);
+
+      const img2 = document.createElement('img');
+      img2.src = 'resources/square20.png';
+      img2.setAttribute('elementtiming', 'my_image2');
+      img2.setAttribute('id', 'visibilityHidden');
+      document.body.appendChild(img2);
+
+      const img3 = document.createElement('img');
+      img3.src = 'resources/circle.svg';
+      img3.setAttribute('elementtiming', 'my_image3');
+      img3.setAttribute('id', 'displayNone');
+      document.body.appendChild(img3);
+
+      const div = document.createElement('div');
+      div.setAttribute('id', 'opacity0');
+      const img4 = document.createElement('img');
+      img4.src = '/images/blue.png';
+      img4.setAttribute('elementtiming', 'my_image4');
+      div.appendChild(img4);
+      document.body.appendChild(div);
+
+      const div2 = document.createElement('div');
+      div2.setAttribute('id', 'visibilityHidden');
+      const img5 = document.createElement('img');
+      img5.src = '/images/blue.png';
+      img5.setAttribute('elementtiming', 'my_image5');
+      div.appendChild(img5);
+      document.body.appendChild(div2);
+
+      const div3 = document.createElement('div');
+      div3.setAttribute('id', 'displayNone');
+      const img6 = document.createElement('img');
+      img6.src = '/images/blue.png';
+      img6.setAttribute('elementtiming', 'my_image6');
+      div.appendChild(img6);
+      document.body.appendChild(div3);
+      // Images have been added but should not cause entries to be dispatched.
+      // Wait for 500ms and end test, ensuring no entry was created.
+      t.step_timeout(() => {
+        t.done();
+      }, 500);
+    };
+  }, 'Images with opacity: 0, visibility: hidden, or display: none are not observable.');
+</script>
diff --git a/testing/web-platform/tests/element-timing/multiple-background-images.html b/testing/web-platform/tests/element-timing/multiple-background-images.html
new file mode 100644
index 0000000000..380e5e825e
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/multiple-background-images.html
@@ -0,0 +1,58 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe element with multiple background images</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#target {
+  width: 200px;
+  height: 200px;
+  background-image: url('resources/circle.svg'), url('resources/square100.png');
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let numObservedElements = 0;
+    let observedCircle = false;
+    let observedSquare = false;
+    const pathPrefix = window.location.origin + '/element-timing/resources/';
+    let div = document.getElementById('target');
+    const observer = new PerformanceObserver(
+      t.step_func(entryList => {
+        entryList.getEntries().forEach(entry => {
+          numObservedElements++;
+          if (entry.url.endsWith('square100.png')) {
+            observedSquare = true;
+            checkElement(entry, pathPrefix + 'square100.png', 'multi', 'target', beforeRender, div);
+            checkRect(entry, [0, 200, 0, 200]);
+            checkNaturalSize(entry, 100, 100);
+          }
+          else if (entry.url.endsWith('circle.svg')) {
+            observedCircle = true;
+            checkElement(entry, pathPrefix + 'circle.svg', 'multi', 'target', beforeRender, div);
+            checkRect(entry, [0, 200, 0, 200]);
+            checkNaturalSize(entry, 200, 200);
+          }
+          else {
+            assert_unreached("Should not have observed an entry with different url!");
+          }
+          if (numObservedElements === 2) {
+            assert_true(observedCircle);
+            assert_true(observedSquare);
+            t.done();
+          }
+        });
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Element with two background images receives both.');
+</script>
+<div id='target' elementtiming='multi'></div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/multiple-redirects-TAO.html b/testing/web-platform/tests/element-timing/multiple-redirects-TAO.html
new file mode 100644
index 0000000000..3a45b552be
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/multiple-redirects-TAO.html
@@ -0,0 +1,62 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<meta charset="utf-8" />
+<title>This test validates some Timing-Allow-Origin header usage in multiple redirects.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+</head>
+<body>
+<script>
+async_test(t => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let destUrl = get_host_info().HTTP_REMOTE_ORIGIN
+        + '/element-timing/resources/multiple-redirects.py?';
+    destUrl += 'redirect_count=2';
+    // The final resource has '*' in TAO header, so will not affect the result.
+    destUrl += '&final_resource=/element-timing/resources/circle-tao.svg';
+    destUrl += '&origin1=' + get_host_info().UNAUTHENTICATED_ORIGIN;
+    destUrl += '&origin2=' + get_host_info().HTTP_REMOTE_ORIGIN;
+    const taoCombinations = [
+        {tao1: location.origin, tao2: location.origin, passes: false},
+        {tao1: location.origin, tao2: get_host_info().HTTP_REMOTE_ORIGIN, passes: false},
+        {tao1: location.origin, tao2: 'null', passes: true},
+        {tao1: location.origin, tao2: '*', passes: true},
+        {tao1: location.origin, tao2: location.origin, passes: false},
+        {tao1: 'null', tao2: '*', passes: false},
+        {tao1: '*', tao2: 'null', passes: true},
+    ];
+    function getURL(item) {
+        return destUrl + '&tao1=' + item.tao1 + '&tao2=' + item.tao2;
+    }
+    taoCombinations.forEach((item, index) => {
+        const image = document.createElement('img');
+        image.src = getURL(item);
+        image.setAttribute('elementtiming', item.passes.toString());
+        image.setAttribute('id', index);
+        document.body.appendChild(image);
+    });
+    let observed = new Array(taoCombinations.length).fill(false);
+    let observedCount = 0;
+    const beforeRender = performance.now();
+    new PerformanceObserver(t.step_func(entries => {
+        entries.getEntries().forEach(e => {
+            const index = parseInt(e.id);
+            assert_false(observed[index]);
+            const item = taoCombinations[index];
+            const url = getURL(item);
+            checkElement(e, url, item.passes.toString(), e.id, item.passes ? beforeRender : 0,
+                document.getElementById(e.id));
+            observed[index] = true;
+            observedCount++;
+            if (observedCount === taoCombinations.length)
+                t.done();
+        });
+    })).observe({entryTypes: ['element']});
+}, 'Cross-origin images with passing/failing TAO should/shouldn\'t have its renderTime set.');
+</script>
+</body>
+</html>
+
diff --git a/testing/web-platform/tests/element-timing/observe-background-image.html b/testing/web-platform/tests/element-timing/observe-background-image.html
new file mode 100644
index 0000000000..6a43401cd0
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-background-image.html
@@ -0,0 +1,37 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe element with background image</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#target {
+  width: 100px;
+  height: 50px;
+  background-image: url('/images/black-rectangle.png');
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender = performance.now();
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/images/black-rectangle.png';
+        checkElement(entry, pathname, 'my_div', 'target', beforeRender,
+            document.getElementById('target'));
+        checkRect(entry, [0, 100, 0, 50]);
+        checkNaturalSize(entry, 100, 50);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+  }, 'Element with elementtiming attribute and background image is observable.');
+</script>
+<div id='target' elementtiming='my_div'></div>
+</body>
diff --git a/testing/web-platform/tests/element-timing/observe-child-element.html b/testing/web-platform/tests/element-timing/observe-child-element.html
new file mode 100644
index 0000000000..c8071998c5
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-child-element.html
@@ -0,0 +1,41 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: do NOT observe elements from same-origin iframes</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done((entryList) => {
+        assert_unreached("Should not have received an entry!");
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the iframe during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    // TODO(npm): change observer to use buffered flag.
+    window.onload = () => {
+      // Add iframe with an image of width and height equal to 100.
+      const iframe = document.createElement('iframe');
+      iframe.src = 'resources/iframe-with-square.html';
+      iframe.onload = () => {
+        // After a short delay, assume that the entry was not dispatched to the
+        // parent frame.
+        t.step_timeout(() => {
+          t.done();
+        }, 100);
+      }
+      document.body.appendChild(iframe);
+    };
+  }, 'Element in child iframe is not observed, even if same-origin.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/observe-elementtiming.html b/testing/web-platform/tests/element-timing/observe-elementtiming.html
new file mode 100644
index 0000000000..a204f0d677
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-elementtiming.html
@@ -0,0 +1,44 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe elements with elementtiming attribute</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender;
+  let img;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/resources/square100.png';
+        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);
+        // Assume viewport has size at least 100, so the element is fully visible.
+        checkRect(entry, [0, 100, 0, 100]);
+        checkNaturalSize(entry, 100, 100);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width equal to 100 and height equal to 100.
+      img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'my_id');
+      document.body.appendChild(img);
+      beforeRender = performance.now();
+    };
+  }, 'Element with elementtiming attribute is observable.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/observe-empty-attribute.html b/testing/web-platform/tests/element-timing/observe-empty-attribute.html
new file mode 100644
index 0000000000..baec6ee48d
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-empty-attribute.html
@@ -0,0 +1,38 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe with empty elementtiming attribute</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+let beforeRender;
+async_test(function (t) {
+  assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+  let observedImage = false;
+  let observedText = false;
+  const observer = new PerformanceObserver(
+    t.step_func(function(entryList) {
+      entryList.getEntries().forEach(entry => {
+        if (entry.name === 'image-paint') {
+          assert_false(observedImage, 'Image should only be observed once.');
+          const pathname = window.location.origin + '/element-timing/resources/square20.png';
+          checkElement(entry, pathname, '', 'square', beforeRender,
+              document.getElementById('square'));
+          checkNaturalSize(entry, 20, 20);
+          observedImage = true;
+        } else {
+          assert_false(observedText, 'Text should only be observed once.');
+          checkTextElement(entry, '', 'text', beforeRender, document.getElementById('text'));
+          observedText = true;
+        }
+      });
+      if (observedImage && observedText)
+        t.done();
+    })
+  );
+  observer.observe({type: 'element', buffered: true});
+  beforeRender = performance.now();
+}, "Able to observe image and text with empty elementtiming attribute.");
+</script>
+<img id='square' src='resources/square20.png' elementtiming/>
+<p id='text' elementtiming>Text!</p>
diff --git a/testing/web-platform/tests/element-timing/observe-multiple-images.html b/testing/web-platform/tests/element-timing/observe-multiple-images.html
new file mode 100644
index 0000000000..c5ea700553
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-multiple-images.html
@@ -0,0 +1,122 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: multiple images</title>
+<body>
+<style>
+body {
+  margin: 0;
+}
+#img1 {
+  display: block;
+  margin-left: auto;
+  margin-right: auto;
+}
+#img2 {
+  margin-top:150px;
+  margin-left:50px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender, image1Observed=0, image2Observed=0, image3Observed=0;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func(function(entryList) {
+        entryList.getEntries().forEach( entry => {
+          if (entry.identifier === 'image1') {
+            if (image1Observed) {
+              assert_unreached("Observer received image1 more than once");
+              t.done();
+            }
+            image1Observed = 1;
+            const pathname1 = window.location.origin + '/element-timing/resources/square100.png';
+            // The images do not contain ID, so expect an empty ID.
+            checkElement(entry, pathname1, 'image1', 'img1', beforeRender,
+                document.getElementById('img1'));
+            // This image is horizontally centered.
+            // Using abs and comparing to 1 because the viewport sizes could be odd.
+            // If a size is odd, then image cannot be in the pure center, but left
+            // and right should still be very close to their estimated coordinates.
+            assert_less_than_equal(Math.abs(entry.intersectionRect.left -
+              (document.documentElement.clientWidth / 2 - 50)), 1,
+              'left of rect for image1');
+            assert_less_than_equal(Math.abs(entry.intersectionRect.right -
+              (document.documentElement.clientWidth / 2 + 50)), 1,
+              'right of rect for image1');
+            assert_equals(entry.intersectionRect.top, 0, 'top of rect for image1');
+            assert_equals(entry.intersectionRect.bottom,
+              100, 'bottom of rect for image1');
+            checkNaturalSize(entry, 100, 100);
+          }
+          else if (entry.identifier === 'image2') {
+            if (image2Observed) {
+              assert_unreached("Observer received image2 more than once");
+              t.done();
+            }
+            image2Observed = 1;
+            const pathname2 = window.location.origin + '/element-timing/resources/square20.png';
+            checkElement(entry, pathname2, 'image2', 'img2', beforeRender,
+                document.getElementById('img2'));
+            // This image should be below image 1, and should respect the margin.
+            checkRect(entry, [50, 250, 250, 450], "of image2");
+            checkNaturalSize(entry, 20, 20);
+          }
+          else if (entry.identifier === 'image3') {
+            if (image3Observed) {
+              assert_unreached("Observer received image3 more than once");
+              t.done();
+            }
+            image3Observed = 1;
+            const pathname3 = window.location.origin + '/element-timing/resources/circle.svg';
+            checkElement(entry, pathname3, 'image3', 'img3', beforeRender,
+                document.getElementById('img3'));
+            // This image is just to the right of image2.
+            checkRect(entry, [250, 450, 250, 450], "of image3");
+            checkNaturalSize(entry, 200, 200);
+          }
+          else {
+            assert_unreached("Received an unexpected identifier.");
+            t.done();
+          }
+          if (image1Observed && image2Observed && image3Observed) {
+            t.done();
+          }
+        });
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    function addImage(number, source, width=0) {
+      const img = document.createElement('img');
+      img.src = source;
+      // Set a different id and elementtiming value for each image.
+      img.id = 'img' + number;
+      img.setAttribute('elementtiming', 'image' + number);
+      if (width !== 0)
+        img.width = width;
+      document.body.appendChild(img);
+    }
+    // Add the images during onload to be sure that the observer is registered in
+    // time to observe the element timing.
+    window.onload = () => {
+      addImage(1, 'resources/square100.png');
+      // Use requestAnimationFrame and a timeout to ensure that the images are
+      // processed in the order we want.
+      requestAnimationFrame( () => {
+        t.step_timeout( () => {
+          // Set the size equal to that of image3 to make positioning easier.
+          addImage(2, 'resources/square20.png', 200);
+          requestAnimationFrame( () => {
+            t.step_timeout( () => {
+              addImage(3, 'resources/circle.svg');
+            }, 0);
+          });
+        }, 0);
+      });
+      beforeRender = performance.now();
+    };
+  }, 'PerformanceObserver can observe multiple image elements.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/observe-shadow-image.html b/testing/web-platform/tests/element-timing/observe-shadow-image.html
new file mode 100644
index 0000000000..e2a81d6244
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-shadow-image.html
@@ -0,0 +1,38 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: do not observe image in shadow tree</title>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<div id='target'></div>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_unreached('Should not observe elements in shadow trees!');
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width equal to 100 and height equal to 100.
+      const img = document.createElement('img');
+      img.src = 'resources/square100.png';
+      img.setAttribute('elementtiming', 'my_image');
+      img.setAttribute('id', 'my_id');
+      const shadowRoot = document.getElementById('target').attachShadow({mode: 'open'});
+      shadowRoot.appendChild(img);
+      t.step_timeout(() => {
+        // Assume entry was not dispatched, so test passes.
+        t.done();
+      }, 500);
+    };
+  }, 'Image in shadow tree with elementtiming attribute is not observable.');
+</script>
diff --git a/testing/web-platform/tests/element-timing/observe-shadow-text.html b/testing/web-platform/tests/element-timing/observe-shadow-text.html
new file mode 100644
index 0000000000..6e6347e60d
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-shadow-text.html
@@ -0,0 +1,37 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: do not observe text in shadow tree</title>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<div id='target'></div>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_unreached('Should not observe text elements in shadow trees!');
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the text during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add text of width equal to 100 and height equal to 100.
+      const text = document.createElement('p');
+      text.innerHTML = 'Text';
+      text.setAttribute('elementtiming', 'my_text');
+      const shadowRoot = document.getElementById('target').attachShadow({mode: 'open'});
+      shadowRoot.appendChild(text);
+      t.step_timeout(() => {
+        // Assume entry was not dispatched, so test passes.
+        t.done();
+      }, 500);
+    };
+  }, 'Text in shadow tree with elementtiming attribute is not observable.');
+</script>
diff --git a/testing/web-platform/tests/element-timing/observe-svg-image.html b/testing/web-platform/tests/element-timing/observe-svg-image.html
new file mode 100644
index 0000000000..737f94f92b
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-svg-image.html
@@ -0,0 +1,34 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe image inside SVG</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+let beforeRender;
+async_test(function (t) {
+  assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+  const observer = new PerformanceObserver(
+    t.step_func_done(function(entryList) {
+      assert_equals(entryList.getEntries().length, 1);
+      const entry = entryList.getEntries()[0];
+      const pathname = window.location.origin + '/element-timing/resources/circle.svg';
+      checkElement(entry, pathname, 'my_svg', 'svg_id', beforeRender,
+          document.getElementById('svg_id'));
+      // Assume viewport has size at least 200, so the element is fully visible.
+      checkRect(entry, [0, 200, 0, 200]);
+      checkNaturalSize(entry, 200, 200);
+    })
+  );
+  observer.observe({entryTypes: ['element']});
+  beforeRender = performance.now();
+}, "Able to observe svg image.");
+</script>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<svg width="300" height="300">
+  <image href='resources/circle.svg' elementtiming='my_svg' id='svg_id'/>
+</svg>
diff --git a/testing/web-platform/tests/element-timing/observe-text.html b/testing/web-platform/tests/element-timing/observe-text.html
new file mode 100644
index 0000000000..5d8955269f
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-text.html
@@ -0,0 +1,48 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe text</title>
+<body>
+<style>
+body {
+  margin: 20px;
+}
+p {
+  font-size: 12px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let paragraph;
+    let beforeRender;
+    const observer = new PerformanceObserver(
+      t.step_func_done((entryList) => {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        checkTextElement(entry, 'my_text', 'text_id', beforeRender, paragraph);
+        assert_equals(entry.intersectionRect.left, 20, 'left should be 20.');
+        // Text box size will vary from device to device, so try lower bounding height by 12, width by 100.
+        assert_greater_than_equal(entry.intersectionRect.right, 120);
+
+        assert_equals(entry.intersectionRect.top, 20, 'top should be 20.');
+        assert_greater_than_equal(entry.intersectionRect.bottom, 32);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the iframe during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      paragraph = document.createElement('p');
+      paragraph.innerHTML = 'This is text I care about';
+      paragraph.setAttribute('elementtiming', 'my_text');
+      paragraph.setAttribute('id', 'text_id');
+      document.body.appendChild(paragraph);
+      beforeRender = performance.now();
+    };
+  }, 'Paragraph with elementtiming attribute is observed.');
+</script>
+
+</body>
diff --git a/testing/web-platform/tests/element-timing/observe-video-poster.html b/testing/web-platform/tests/element-timing/observe-video-poster.html
new file mode 100644
index 0000000000..5607733529
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/observe-video-poster.html
@@ -0,0 +1,32 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe video poster image</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+let beforeRender;
+async_test(function (t) {
+  assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+  const observer = new PerformanceObserver(
+    t.step_func_done(function(entryList) {
+      assert_equals(entryList.getEntries().length, 1);
+      const entry = entryList.getEntries()[0];
+      const pathname = window.location.origin + '/element-timing/resources/circle.svg';
+      checkElement(entry, pathname, 'my_poster', 'the_poster', beforeRender,
+          document.getElementById('the_poster'));
+      // Assume viewport has size at least 200, so the element is fully visible.
+      checkRect(entry, [0, 200, 0, 200]);
+      checkNaturalSize(entry, 200, 200);
+    })
+  );
+  observer.observe({entryTypes: ['element']});
+  beforeRender = performance.now();
+}, "Able to observe a video's poster image.");
+</script>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<video elementtiming='my_poster' id='the_poster' src='/media/test.mp4' poster='resources/circle.svg'/>
diff --git a/testing/web-platform/tests/element-timing/progressively-loaded-image.html b/testing/web-platform/tests/element-timing/progressively-loaded-image.html
new file mode 100644
index 0000000000..6695d8f9c3
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/progressively-loaded-image.html
@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html>
+<meta charset=utf-8 />
+<title>Element Timing: buffer elements before onload</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<script src="resources/element-timing-helpers.js"></script>
+<body>
+<script>
+  let beforeRender;
+  let img;
+  // Number of characters to be read on the initial read, before sleeping.
+  // Should be sufficient to do at least a first scan.
+  let numInitial = 75;
+  let sleep = 500;
+  async_test(function(t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const img_src = 'resources/progressive-image.py?name=square20.jpg&numInitial='
+      + numInitial + '&sleep=' + sleep;
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/element-timing/' + img_src;
+        // Since the image is only fully loaded after the sleep, the render timestamp
+        // must be greater than |beforeRender| + |sleep|.
+        checkElement(entry, pathname, 'my_image', '', beforeRender + sleep, img);
+        checkNaturalSize(entry, 20, 20);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+
+    img = document.createElement('img');
+    img.src = img_src;
+    img.setAttribute('elementtiming', 'my_image');
+    document.body.appendChild(img);
+    beforeRender = performance.now();
+    t.step_timeout(() => {assert_true(0);}, 2000);
+  }, "Element Timing: image render timestamp occurs after it is fully loaded.");
+</script>
diff --git a/testing/web-platform/tests/element-timing/rectangular-image.html b/testing/web-platform/tests/element-timing/rectangular-image.html
new file mode 100644
index 0000000000..65f190e753
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/rectangular-image.html
@@ -0,0 +1,43 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe a rectangular image</title>
+<body>
+<style>
+body {
+  margin: 20px;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let beforeRender;
+  let img;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        const pathname = window.location.origin + '/images/black-rectangle.png';
+        checkElement(entry, pathname, 'my_image', 'rectangle', beforeRender, img);
+        // Assume viewport has size at least 100, so the element is fully visible.
+        checkRect(entry, [20, 120, 20, 70]);
+        checkNaturalSize(entry, 100, 50);
+      })
+    );
+    observer.observe({entryTypes: ['element']});
+    // We add the image during onload to be sure that the observer is registered
+    // in time for it to observe the element timing.
+    window.onload = () => {
+      // Add image of width equal to 100 and height equal to 50.
+      img = document.createElement('img');
+      img.src = '/images/black-rectangle.png';
+      img.id = 'rectangle';
+      img.setAttribute('elementtiming', 'my_image');
+      document.body.appendChild(img);
+      beforeRender = performance.now();
+    };
+  }, 'Element with rectangular image has correct rect and instrinsic size.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/redirects-tao-star.html b/testing/web-platform/tests/element-timing/redirects-tao-star.html
new file mode 100644
index 0000000000..e5067d3d6b
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/redirects-tao-star.html
@@ -0,0 +1,53 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<meta charset="utf-8" />
+<title>This test validates element timing information for cross-origin redirect chain images.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script src=/common/get-host-info.sub.js></script>
+</head>
+<body>
+<script>
+async_test(t => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    let destUrl = get_host_info().HTTP_REMOTE_ORIGIN
+            + '/resource-timing/resources/multi_redirect.py?';
+    destUrl += 'page_origin=' +  get_host_info().HTTP_ORIGIN;
+    destUrl += '&cross_origin=' + get_host_info().HTTP_REMOTE_ORIGIN;
+    destUrl += '&final_resource=' + '/element-timing/resources/circle-tao.svg';
+    destUrl += '&timing_allow=1';
+    destUrl += '&tao_steps=';
+    for (let taoSteps=0; taoSteps < 4; taoSteps++) {
+        const image = document.createElement('img');
+        image.src = destUrl + taoSteps;
+        image.setAttribute('elementtiming', taoSteps);
+        image.setAttribute('id', 'id' + taoSteps)
+        document.body.appendChild(image);
+    }
+    let numObserved = 0;
+    let observedMap = [false, false, false, false];
+    const beforeRender = performance.now();
+    new PerformanceObserver(t.step_func(entries => {
+        entries.getEntries().forEach(entry => {
+            const taoSteps = entry.identifier;
+            assert_false(observedMap[taoSteps], 'Should observe each image once');
+            observedMap[taoSteps] = true;
+            if (taoSteps !== '3') {
+                assert_equals(entry.renderTime, 0,
+                    'renderTime should be 0 when there is ' + taoSteps + ' tao steps');
+            }
+            const bound = taoSteps === '3' ? beforeRender : 0;
+            checkElement(entry, destUrl + taoSteps, taoSteps, 'id' + taoSteps, bound,
+                    document.getElementById(taoSteps));
+        });
+        numObserved += entries.getEntries().length;
+        if (numObserved === 4)
+            t.done();
+    })).observe({type: 'element', buffered: true});
+}, 'Cross-origin image without TAO should not have its renderTime set, with full TAO it should.');
+</script>
+</body>
+</html>
+
diff --git a/testing/web-platform/tests/element-timing/resources/TAOImage.py b/testing/web-platform/tests/element-timing/resources/TAOImage.py
new file mode 100644
index 0000000000..0e1c481489
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/TAOImage.py
@@ -0,0 +1,47 @@
+import os
+
+from wptserve.utils import isomorphic_encode
+
+def main(request, response):
+    origin = request.GET.first(b'origin')
+    if origin:
+        response.headers.set(b'Access-Control-Allow-Origin', origin)
+
+    tao = request.GET.first(b'tao')
+
+    if tao == b'wildcard':
+    # wildcard, pass
+        response.headers.set(b'Timing-Allow-Origin', b'*')
+    elif tao == b'null':
+    # null, fail
+        response.headers.set(b'Timing-Allow-Origin', b'null')
+    elif tao == b'origin':
+    # case-sensitive match for origin, pass
+        response.headers.set(b'Timing-Allow-Origin', origin)
+    elif tao == b'space':
+    # space separated list of origin and wildcard, fail
+        response.headers.set(b'Timing-Allow-Origin', (origin + b' *'))
+    elif tao == b'multi':
+    # more than one TAO values, separated by comma, pass
+        response.headers.set(b'Timing-Allow-Origin', origin)
+        response.headers.append(b'Timing-Allow-Origin', b'*')
+    elif tao == b'multi_wildcard':
+    # multiple wildcards, separated by comma, pass
+        response.headers.set(b'Timing-Allow-Origin', b'*')
+        response.headers.append(b'Timing-Allow-Origin', b'*')
+    elif tao == b'match_origin':
+    # contains a match of origin, separated by comma, pass
+        response.headers.set(b'Timing-Allow-Origin', origin)
+        response.headers.append(b'Timing-Allow-Origin', b"fake")
+    elif tao == b'match_wildcard':
+    # contains a wildcard, separated by comma, pass
+        response.headers.set(b'Timing-Allow-Origin', b"fake")
+        response.headers.append(b'Timing-Allow-Origin', b'*')
+    elif tao == b'uppercase':
+    # non-case-sensitive match for origin, fail
+        response.headers.set(b'Timing-Allow-Origin', origin.upper())
+    else:
+        pass
+    response.headers.set(b"Cache-Control", b"no-cache, must-revalidate");
+    image_path = os.path.join(os.path.dirname(isomorphic_encode(__file__)), b"square100.png");
+    response.content = open(image_path, mode=u'rb').read();
diff --git a/testing/web-platform/tests/element-timing/resources/circle-tao.svg b/testing/web-platform/tests/element-timing/resources/circle-tao.svg
new file mode 100644
index 0000000000..209b9f4e5b
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/circle-tao.svg
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
+              "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
+<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
+    <circle cx="50%" cy="50%" r="80" style="fill:blue;" />
+</svg>
diff --git a/testing/web-platform/tests/element-timing/resources/circle-tao.svg.headers b/testing/web-platform/tests/element-timing/resources/circle-tao.svg.headers
new file mode 100644
index 0000000000..7296361df3
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/circle-tao.svg.headers
@@ -0,0 +1 @@
+Timing-Allow-Origin: *
diff --git a/testing/web-platform/tests/element-timing/resources/circle.svg b/testing/web-platform/tests/element-timing/resources/circle.svg
new file mode 100644
index 0000000000..209b9f4e5b
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/circle.svg
@@ -0,0 +1,6 @@
+<?xml version="1.0"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN"
+              "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
+<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
+    <circle cx="50%" cy="50%" r="80" style="fill:blue;" />
+</svg>
diff --git a/testing/web-platform/tests/element-timing/resources/element-timing-helpers.js b/testing/web-platform/tests/element-timing/resources/element-timing-helpers.js
new file mode 100644
index 0000000000..8175b0f385
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/element-timing-helpers.js
@@ -0,0 +1,74 @@
+// Common checks between checkElement() and checkElementWithoutResourceTiming().
+function checkElementInternal(entry, expectedUrl, expectedIdentifier, expectedID, beforeRender,
+    expectedElement) {
+  assert_equals(entry.entryType, 'element', 'entryType does not match');
+  assert_equals(entry.url, expectedUrl, 'url does not match');
+  assert_equals(entry.identifier, expectedIdentifier, 'identifier does not match');
+  if (beforeRender != 0) {
+    // In this case, renderTime is not 0.
+    assert_greater_than(entry.renderTime, 0, 'renderTime should be nonzero');
+    assert_equals(entry.startTime, entry.renderTime, 'startTime should equal renderTime');
+  } else {
+    // In this case, renderTime is 0, so compare to loadTime.
+    assert_equals(entry.renderTime, 0, 'renderTime should be zero');
+    assert_equals(entry.startTime, entry.loadTime, 'startTime should equal loadTime');
+  }
+  assert_equals(entry.duration, 0, 'duration should be 0');
+  assert_equals(entry.id, expectedID, 'id does not match');
+  assert_greater_than_equal(entry.renderTime, beforeRender, 'renderTime greater than beforeRender');
+  assert_greater_than_equal(performance.now(), entry.renderTime, 'renderTime bounded by now()');
+  if (expectedElement !== null) {
+    assert_equals(entry.element, expectedElement, 'element does not match');
+    assert_equals(entry.identifier, expectedElement.elementTiming,
+        'identifier must be the elementtiming of the element');
+    assert_equals(entry.id, expectedElement.id, 'id must be the id of the element');
+  }
+}
+
+// Checks that this is an ElementTiming entry with url |expectedUrl|. It also
+// does a very basic check on |renderTime|: after |beforeRender| and before now().
+function checkElement(entry, expectedUrl, expectedIdentifier, expectedID, beforeRender,
+    expectedElement) {
+  checkElementInternal(entry, expectedUrl, expectedIdentifier, expectedID, beforeRender,
+      expectedElement);
+  assert_equals(entry.name, 'image-paint', 'The entry name should be image-paint.');
+  const rt_entries = performance.getEntriesByName(expectedUrl, 'resource');
+  assert_equals(rt_entries.length, 1, 'There should be only 1 resource entry.');
+  assert_greater_than_equal(entry.loadTime, rt_entries[0].responseEnd,
+    'Image loadTime is after the resource responseEnd');
+}
+
+function checkElementWithoutResourceTiming(entry, expectedUrl, expectedIdentifier,
+    expectedID, beforeRender, expectedElement) {
+  checkElementInternal(entry, expectedUrl, expectedIdentifier, expectedID, beforeRender,
+      expectedElement);
+  assert_equals(entry.name, 'image-paint');
+  // No associated resource from ResourceTiming, so not much to compare loadTime with.
+  assert_greater_than(entry.loadTime, 0, 'The entry loadTime should be greater than 0.');
+}
+
+// Checks that the rect matches the desired values [left right top bottom].
+function checkRect(entry, expected, description="") {
+  assert_equals(entry.intersectionRect.left, expected[0],
+    'left of rect ' + description);
+  assert_equals(entry.intersectionRect.right, expected[1],
+    'right of rect ' + description);
+  assert_equals(entry.intersectionRect.top, expected[2],
+    'top of rect ' + description);
+  assert_equals(entry.intersectionRect.bottom, expected[3],
+    'bottom of rect ' + description);
+}
+
+// Checks that the intrinsic size matches the desired values.
+function checkNaturalSize(entry, width, height) {
+  assert_equals(entry.naturalWidth, width, 'The entry naturalWidth is not as expected.');
+  assert_equals(entry.naturalHeight, height, 'The entry naturalHeight is not as expected.');
+}
+
+function checkTextElement(entry, expectedIdentifier, expectedID, beforeRender,
+    expectedElement) {
+  checkElementInternal(entry, '', expectedIdentifier, expectedID, beforeRender,
+      expectedElement);
+  assert_equals(entry.name, 'text-paint', 'The entry name should be text-paint.');
+  assert_equals(entry.loadTime, 0, 'The entry loadTime should be 0.');
+}
diff --git a/testing/web-platform/tests/element-timing/resources/iframe-stores-entry.html b/testing/web-platform/tests/element-timing/resources/iframe-stores-entry.html
new file mode 100644
index 0000000000..2fa2476972
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/iframe-stores-entry.html
@@ -0,0 +1,12 @@
+<!DOCTYPE html>
+<html>
+<body>
+<p elementtiming='text'>Text</p>
+<script>
+  const observer = new PerformanceObserver(entryList => {
+    window.parent.triggerTest(entryList.getEntries()[0]);
+  });
+  observer.observe({type: 'element', buffered: true});
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/element-timing/resources/iframe-with-content.html b/testing/web-platform/tests/element-timing/resources/iframe-with-content.html
new file mode 100644
index 0000000000..ab8cdfde01
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/iframe-with-content.html
@@ -0,0 +1,5 @@
+<!DOCTYPE html>
+<body>
+<p>Text</p>
+<img src='/images/blue.png'/>
+</body>
diff --git a/testing/web-platform/tests/element-timing/resources/iframe-with-square-sends-entry.html b/testing/web-platform/tests/element-timing/resources/iframe-with-square-sends-entry.html
new file mode 100644
index 0000000000..b8af505d32
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/iframe-with-square-sends-entry.html
@@ -0,0 +1,26 @@
+<!DOCType html>
+<html>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<body>
+<script>
+  const observer = new PerformanceObserver(entryList => {
+    top.postMessage({
+      'length' : entryList.getEntries().length,
+      'entryType' : entryList.getEntries()[0].entryType,
+      'rect' : entryList.getEntries()[0].intersectionRect,
+      'naturalWidth' : entryList.getEntries()[0].naturalWidth,
+      'naturalHeight' : entryList.getEntries()[0].naturalHeight,
+      'id': entryList.getEntries()[0].id,
+      // Elements cannot be cloned, so just send the element ID.
+      'elementId' : entryList.getEntries()[0].element.id,
+    }, '*');
+  });
+  observer.observe({entryTypes: ['element']});
+</script>
+<img src=square100.png id=iframe_img_id elementtiming=my_image/>
+</body>
+</html>
diff --git a/testing/web-platform/tests/element-timing/resources/iframe-with-square.html b/testing/web-platform/tests/element-timing/resources/iframe-with-square.html
new file mode 100644
index 0000000000..0a905d22b9
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/iframe-with-square.html
@@ -0,0 +1,10 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<body>
+<style>
+body {
+  margin: 0;
+}
+</style>
+<img src='square100.png' elementtiming="my_image">
+</body>
diff --git a/testing/web-platform/tests/element-timing/resources/multiple-redirects.py b/testing/web-platform/tests/element-timing/resources/multiple-redirects.py
new file mode 100644
index 0000000000..3efc634b99
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/multiple-redirects.py
@@ -0,0 +1,59 @@
+from wptserve.utils import isomorphic_encode
+
+def main(request, response):
+    """Handler that causes multiple redirections.
+
+    Mandatory parameters:
+    redirect_count - A number which is at least 1 (number of redirects).
+    final_resource - The location of the last redirect.
+
+    For each number i between 1 and |redirect_count| we have the following optional parameters:
+    tao{{i}} - The Timing-Allow-Origin header of the ith response. Default is no header.
+    origin{{i}} - The origin of the ith redirect (i+1 response). Default is location.origin.
+    Note that the origin of the initial request cannot be controlled here
+    and the Timing-Allow-Origin header of the final response cannot be controlled here.
+
+    Example: redirect_count=2&final_resource=miau.png&tao1=*
+
+    Note: |step| is used internally to track the current redirect number.
+    """
+    step = 1
+    if b"step" in request.GET:
+        try:
+            step = int(request.GET.first(b"step"))
+        except ValueError:
+            pass
+
+    redirect_count = int(request.GET.first(b"redirect_count"))
+    final_resource = request.GET.first(b"final_resource")
+
+    tao_value = None
+    tao = b"tao" + isomorphic_encode(str(step))
+    if tao in request.GET:
+        tao_value = request.GET.first(tao)
+
+    redirect_url = b""
+    origin = b"origin" + isomorphic_encode(str(step))
+    if origin in request.GET:
+        redirect_url = request.GET.first(origin)
+
+    if step == redirect_count:
+        redirect_url += final_resource
+    else:
+        redirect_url += b"/element-timing/resources/multiple-redirects.py?"
+        redirect_url += b"redirect_count=" + isomorphic_encode(str(redirect_count))
+        redirect_url += b"&final_resource=" + final_resource
+        for i in range(1, redirect_count + 1):
+            tao = b"tao" + isomorphic_encode(str(i))
+            if tao in request.GET:
+                redirect_url += b"&" + tao + b"=" + request.GET.first(tao)
+            origin = b"origin" + isomorphic_encode(str(i))
+            if origin in request.GET:
+                redirect_url += b"&" + origin + b"=" + request.GET.first(origin)
+        redirect_url += b"&step=" + isomorphic_encode(str(step + 1))
+
+    if tao_value:
+        response.headers.set(b"timing-allow-origin", tao_value)
+
+    response.status = 302
+    response.headers.set(b"Location", redirect_url)
diff --git a/testing/web-platform/tests/element-timing/resources/progressive-image.py b/testing/web-platform/tests/element-timing/resources/progressive-image.py
new file mode 100644
index 0000000000..84e98c8451
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/progressive-image.py
@@ -0,0 +1,27 @@
+import os.path
+import time
+
+from wptserve.utils import isomorphic_encode
+
+def main(request, response):
+    name = request.GET.first(b"name")
+    sleepTime = float(request.GET.first(b"sleep")) / 1E3
+    numInitial = int(request.GET.first(b"numInitial"))
+
+    path = os.path.join(os.path.dirname(isomorphic_encode(__file__)), name)
+    body = open(path, u"rb").read()
+
+    response.headers.set(b"Content-Type", b"image")
+    response.headers.set(b"Content-Length", len(body))
+    response.headers.set(b"Cache-control", b"no-cache, must-revalidate")
+    response.write_status_headers()
+
+    # Read from the beginning, |numInitial| bytes.
+    first = body[:numInitial]
+    response.writer.write_content(first)
+
+    time.sleep(sleepTime)
+
+    # Read the remainder after having slept.
+    second = body[numInitial:]
+    response.writer.write_content(second)
diff --git a/testing/web-platform/tests/element-timing/resources/slow-image.py b/testing/web-platform/tests/element-timing/resources/slow-image.py
new file mode 100644
index 0000000000..10172fd35a
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/resources/slow-image.py
@@ -0,0 +1,19 @@
+import os.path
+import time
+
+from wptserve.utils import isomorphic_encode
+
+def main(request, response):
+    name = request.GET.first(b"name")
+    sleepTime = float(request.GET.first(b"sleep")) / 1E3
+
+    time.sleep(sleepTime)
+
+    path = os.path.join(os.path.dirname(isomorphic_encode(__file__)), name)
+    body = open(path, u"rb").read()
+
+    response.headers.set(b"Content-Type", b"image")
+    response.headers.set(b"Content-Length", len(body))
+    response.headers.set(b"Cache-control", b"no-cache, must-revalidate")
+
+    response.content = body;
diff --git a/testing/web-platform/tests/element-timing/resources/square100.png b/testing/web-platform/tests/element-timing/resources/square100.png
new file mode 100644
index 0000000000..567babb96d
Binary files /dev/null and b/testing/web-platform/tests/element-timing/resources/square100.png differ
diff --git a/testing/web-platform/tests/element-timing/resources/square20.jpg b/testing/web-platform/tests/element-timing/resources/square20.jpg
new file mode 100644
index 0000000000..83ed4914bb
Binary files /dev/null and b/testing/web-platform/tests/element-timing/resources/square20.jpg differ
diff --git a/testing/web-platform/tests/element-timing/resources/square20.png b/testing/web-platform/tests/element-timing/resources/square20.png
new file mode 100644
index 0000000000..4d51ac4b46
Binary files /dev/null and b/testing/web-platform/tests/element-timing/resources/square20.png differ
diff --git a/testing/web-platform/tests/element-timing/retrievability.html b/testing/web-platform/tests/element-timing/retrievability.html
new file mode 100644
index 0000000000..cd2c2a956e
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/retrievability.html
@@ -0,0 +1,29 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: 'element' entries are not accessible via performance timeline</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<script>
+  let img;
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const beforeRender = performance.now();
+    new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        assert_equals(entry.entryType, 'element');
+        assert_equals(entry.name, 'image-paint');
+
+        const entriesByName = performance.getEntriesByName('image-paint', 'element');
+        const entriesByType = performance.getEntriesByType('element');
+        const allEntries = performance.getEntries();
+        assert_equals(entriesByName.length, 0, 'Element Timing entry should not be retrievable by getEntriesByName');
+        assert_equals(entriesByType.length, 0, 'Element Timing entry should not be retrievable by getEntriesByType');
+        assert_equals(allEntries.filter(e => e.entryType === 'element').length, 0, 'Element Timing entry should not be retrievable by getEntries');
+      })
+    ).observe({type: 'element', buffered: true});
+  }, 'Element Timing entries are not accessible via performance.getEntries*');
+</script>
+<img src='resources/square100.png' elementtiming='my_image' id='my_id'/>
diff --git a/testing/web-platform/tests/element-timing/same-origin-redirects.html b/testing/web-platform/tests/element-timing/same-origin-redirects.html
new file mode 100644
index 0000000000..e52fcecc1a
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/same-origin-redirects.html
@@ -0,0 +1,37 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<meta charset="utf-8" />
+<title>This test validates element timing information for same-origin redirect chain.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+</head>
+<body>
+<script>
+async_test(t => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    // First redirect
+    let destUrl = '/common/redirect.py?location='
+    // Second redirect
+    destUrl += '/common/redirect.py?location='
+    // Image without TAO headers.
+    destUrl += '/element-timing/resources/square20.png';
+    let beforeRender;
+    new PerformanceObserver(t.step_func_done(entries => {
+        assert_equals(entries.getEntries().length, 1, 'There should be one entry');
+        const entry = entries.getEntries()[0];
+        checkElement(entry, location.origin + destUrl, 'et', 'id', beforeRender,
+                document.getElementById('id'));
+    })).observe({entryTypes: ['element']});
+    const image = document.createElement('img');
+    image.src = destUrl;
+    image.setAttribute('elementtiming', 'et');
+    image.setAttribute('id', 'id')
+    document.body.appendChild(image);
+    beforeRender = performance.now();
+}, 'Same-origin image redirect without TAO should have its renderTime set.');
+</script>
+</body>
+</html>
+
diff --git a/testing/web-platform/tests/element-timing/scroll-to-text.html b/testing/web-platform/tests/element-timing/scroll-to-text.html
new file mode 100644
index 0000000000..0508d2bcf9
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/scroll-to-text.html
@@ -0,0 +1,30 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe text that is initially not visible</title>
+<body>
+<style>
+.big {
+  width: 100%;
+  height: 100vh;
+}
+</style>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<div class='big'></div>
+<p elementtiming='observeMe'>Test text</p>
+<script>
+  async_test((t) => {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(t.step_func_done(() => {}));
+    observer.observe({type: 'element', buffered: true});
+    window.onload = () => {
+      // The div occupies the whole screen because it occupies 100% of the height.
+      // We scroll to the end of the document so that the paragraph becomes visible.
+      // A user agent could paint the text before or after scrolling, but either way
+      // it must produce an entry for it.
+      window.scrollTo(0,document.scrollingElement.scrollHeight);
+    };
+  }, 'Paragraph with elementtiming attribute is observed even when not initially visible.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/supported-element-type.html b/testing/web-platform/tests/element-timing/supported-element-type.html
new file mode 100644
index 0000000000..15733d46f9
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/supported-element-type.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<head>
+<title>PerformanceObserver.supportedEntryTypes contains "element"</title>
+</head>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+test(() => {
+  if (typeof PerformanceObserver.supportedEntryTypes === "undefined")
+    assert_unreached("supportedEntryTypes is not supported.");
+  assert_greater_than(PerformanceObserver.supportedEntryTypes.indexOf("element"), -1,
+    "There should be an entry 'element' in PerformanceObserver.supportedEntryTypes");
+}, "supportedEntryTypes contains 'element'.");
+</script>
+</body>
+</html>
diff --git a/testing/web-platform/tests/element-timing/text-with-display-style.html b/testing/web-platform/tests/element-timing/text-with-display-style.html
new file mode 100644
index 0000000000..94e89fcf72
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/text-with-display-style.html
@@ -0,0 +1,54 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: observe text with display style.</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<style>
+h1 {
+  display: flex;
+}
+h2 {
+  display: grid;
+}
+h3 {
+  display: block;
+}
+</style>
+<h1 id='title1' elementtiming='h1'>I am h1</h1>
+<h2 id='title2' elementtiming='h2'>I am h2</h2>
+<h3 id='title3' elementtiming='h3'>I am h3</h3>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const beforeRender = performance.now();
+    let observedFlex = false;
+    let observedGrid = false;
+    let observedBlock = false;
+    const observer = new PerformanceObserver(
+      t.step_func(function(entryList) {
+        entryList.getEntries().forEach(entry => {
+          if (entry.id === 'title1') {
+            assert_false(observedFlex, 'Should observe flex once');
+            observedFlex = true;
+            checkTextElement(entry, 'h1', 'title1', beforeRender, document.getElementById('title1'));
+          } else if (entry.id === 'title2') {
+            assert_false(observedGrid, 'Should observe grid once');
+            observedGrid = true;
+            checkTextElement(entry, 'h2', 'title2', beforeRender, document.getElementById('title2'));
+          } else {
+            assert_equals(entry.id, 'title3', 'ID should be title1, title2, or title3');
+            assert_false(observedBlock, 'Should observe block once');
+            observedBlock = true;
+            checkTextElement(entry, 'h3', 'title3', beforeRender, document.getElementById('title3'));
+          }
+        });
+        if (observedFlex && observedGrid && observedBlock)
+          t.done();
+      })
+    );
+    observer.observe({type: 'element', buffered: true});
+  }, 'Text with display style is observable.');
+</script>
+</body>
diff --git a/testing/web-platform/tests/element-timing/toJSON.html b/testing/web-platform/tests/element-timing/toJSON.html
new file mode 100644
index 0000000000..a9f6a5180a
--- /dev/null
+++ b/testing/web-platform/tests/element-timing/toJSON.html
@@ -0,0 +1,45 @@
+<!DOCTYPE HTML>
+<meta charset=utf-8>
+<title>Element Timing: toJSON</title>
+<body>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="resources/element-timing-helpers.js"></script>
+<img elementtiming='img' src="resources/square100.png"/>
+<script>
+  async_test(function (t) {
+    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");
+    const observer = new PerformanceObserver(
+      t.step_func_done(function(entryList) {
+        assert_equals(entryList.getEntries().length, 1);
+        const entry = entryList.getEntries()[0];
+        assert_equals(typeof(entry.toJSON), 'function');
+        const json = entry.toJSON();
+        assert_equals(typeof(json), 'object');
+        const keys = [
+          // PerformanceEntry
+          'name',
+          'entryType',
+          'startTime',
+          'duration',
+          // PerformanceElementTiming
+          'renderTime',
+          'loadTime',
+          'intersectionRect',
+          'identifier',
+          'naturalWidth',
+          'naturalHeight',
+          'id',
+          'url',
+        ];
+        for (const key of keys) {
+          assert_equals(json[key], entry[key],
+            'PerformanceElementTiming ${key} entry does not match its toJSON value');
+        }
+        assert_equals(json['element'], undefined, 'toJSON should not include element');
+      })
+    );
+    observer.observe({type: 'element', buffered: true});
+  }, 'Test toJSON() in PerformanceElementTiming.');
+</script>
+</body>
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