From 0ebf5bdf043a27fd3dfb7f92e0cb63d88954c44d Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Fri, 19 Apr 2024 03:47:29 +0200
Subject: Adding upstream version 115.8.0esr.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../web-platform/tests/resize-observer/META.yml    |    4 +
 .../resize-observer/calculate-depth-for-node.html  |   26 +
 .../callback-cross-realm-report-exception.html     |   30 +
 .../resize-observer/change-layout-in-error.html    |   33 +
 .../resize-observer/create-pattern-data-url.js     |   22 +
 .../tests/resize-observer/devicepixel-ref.html     |   27 +
 .../tests/resize-observer/devicepixel.html         |   85 ++
 .../tests/resize-observer/devicepixel2-ref.html    |   21 +
 .../tests/resize-observer/devicepixel2.html        |  131 +++
 .../tests/resize-observer/eventloop.html           |  253 +++++
 .../tests/resize-observer/fragments.html           |  115 +++
 .../tests/resize-observer/idlharness.window.js     |   37 +
 .../resize-observer/iframe-same-origin-ref.html    |    7 +
 .../tests/resize-observer/iframe-same-origin.html  |   76 ++
 .../web-platform/tests/resize-observer/notify.html |  408 ++++++++
 .../tests/resize-observer/observe.html             | 1007 ++++++++++++++++++++
 .../tests/resize-observer/ordering.html            |   24 +
 .../tests/resize-observer/resources/iframe.html    |   38 +
 .../tests/resize-observer/resources/image.png      |  Bin 0 -> 170 bytes
 .../resize-observer/resources/resizeTestHelper.js  |  195 ++++
 .../tests/resize-observer/scrollbars-2.html        |   36 +
 .../tests/resize-observer/scrollbars.html          |   56 ++
 .../resize-observer/svg-with-css-box-001.html      |   91 ++
 .../tests/resize-observer/svg-with-css-box-002.svg |   94 ++
 .../web-platform/tests/resize-observer/svg.html    |  619 ++++++++++++
 25 files changed, 3435 insertions(+)
 create mode 100644 testing/web-platform/tests/resize-observer/META.yml
 create mode 100644 testing/web-platform/tests/resize-observer/calculate-depth-for-node.html
 create mode 100644 testing/web-platform/tests/resize-observer/callback-cross-realm-report-exception.html
 create mode 100644 testing/web-platform/tests/resize-observer/change-layout-in-error.html
 create mode 100644 testing/web-platform/tests/resize-observer/create-pattern-data-url.js
 create mode 100644 testing/web-platform/tests/resize-observer/devicepixel-ref.html
 create mode 100644 testing/web-platform/tests/resize-observer/devicepixel.html
 create mode 100644 testing/web-platform/tests/resize-observer/devicepixel2-ref.html
 create mode 100644 testing/web-platform/tests/resize-observer/devicepixel2.html
 create mode 100644 testing/web-platform/tests/resize-observer/eventloop.html
 create mode 100644 testing/web-platform/tests/resize-observer/fragments.html
 create mode 100644 testing/web-platform/tests/resize-observer/idlharness.window.js
 create mode 100644 testing/web-platform/tests/resize-observer/iframe-same-origin-ref.html
 create mode 100644 testing/web-platform/tests/resize-observer/iframe-same-origin.html
 create mode 100644 testing/web-platform/tests/resize-observer/notify.html
 create mode 100644 testing/web-platform/tests/resize-observer/observe.html
 create mode 100644 testing/web-platform/tests/resize-observer/ordering.html
 create mode 100644 testing/web-platform/tests/resize-observer/resources/iframe.html
 create mode 100644 testing/web-platform/tests/resize-observer/resources/image.png
 create mode 100644 testing/web-platform/tests/resize-observer/resources/resizeTestHelper.js
 create mode 100644 testing/web-platform/tests/resize-observer/scrollbars-2.html
 create mode 100644 testing/web-platform/tests/resize-observer/scrollbars.html
 create mode 100644 testing/web-platform/tests/resize-observer/svg-with-css-box-001.html
 create mode 100644 testing/web-platform/tests/resize-observer/svg-with-css-box-002.svg
 create mode 100644 testing/web-platform/tests/resize-observer/svg.html

(limited to 'testing/web-platform/tests/resize-observer')

diff --git a/testing/web-platform/tests/resize-observer/META.yml b/testing/web-platform/tests/resize-observer/META.yml
new file mode 100644
index 0000000000..40ebbe9662
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/META.yml
@@ -0,0 +1,4 @@
+spec: https://drafts.csswg.org/resize-observer/
+suggested_reviewers:
+  - atotic
+  - dholbert
diff --git a/testing/web-platform/tests/resize-observer/calculate-depth-for-node.html b/testing/web-platform/tests/resize-observer/calculate-depth-for-node.html
new file mode 100644
index 0000000000..339e52bb79
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/calculate-depth-for-node.html
@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="help" href="https://drafts.csswg.org/resize-observer/#calculate-depth-for-node">
+<body>
+<div id="host"></div>
+<script>
+let didSeeError = false;
+window.onerror = (message, source, lineno, colno, error) => {
+  didSeeError = true;
+}
+
+async_test(t => {
+  let host = document.querySelector('#host');
+  let observer = new ResizeObserver(t.step_func(() => {
+    let root = host.attachShadow({mode:'open'});
+    let child = root.appendChild(document.createElement('div'));
+    new ResizeObserver(() => {}).observe(child);
+    requestAnimationFrame(t.step_func_done(() => { assert_false(didSeeError); }));
+  }));
+  observer.observe(host);
+}, '"Calculate depth for node" algorithm with Shadow DOM');
+
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/resize-observer/callback-cross-realm-report-exception.html b/testing/web-platform/tests/resize-observer/callback-cross-realm-report-exception.html
new file mode 100644
index 0000000000..75a91ec0a1
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/callback-cross-realm-report-exception.html
@@ -0,0 +1,30 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>ResizeObserver reports the exception from its callback in the callback's global object</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<iframe srcdoc='<div style="height: 100px;">foo</div>'></iframe>
+<iframe></iframe>
+<iframe></iframe>
+<script>
+setup({ allow_uncaught_exception: true });
+
+const onerrorCalls = [];
+window.onerror = () => { onerrorCalls.push("top"); };
+frames[0].onerror = () => { onerrorCalls.push("frame0"); };
+frames[1].onerror = () => { onerrorCalls.push("frame1"); };
+frames[2].onerror = () => { onerrorCalls.push("frame2"); };
+
+async_test(t => {
+  window.onload = t.step_func(() => {
+    const target = frames[0].document.querySelector("div");
+    const io = new frames[0].ResizeObserver(new frames[1].Function(`throw new parent.frames[2].Error("PASS");`));
+    io.observe(target);
+
+    t.step_timeout(() => {
+      assert_array_equals(onerrorCalls, ["frame1"]);
+      t.done();
+    }, 25);
+  });
+});
+</script>
diff --git a/testing/web-platform/tests/resize-observer/change-layout-in-error.html b/testing/web-platform/tests/resize-observer/change-layout-in-error.html
new file mode 100644
index 0000000000..9083fb48f9
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/change-layout-in-error.html
@@ -0,0 +1,33 @@
+<!doctype HTML>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<style>
+  div {
+    width: 100px;
+    height: 100px;
+  }
+</style>
+<div id="observeme"></div>
+
+<script>
+setup({allow_uncaught_exception: true});
+
+async_test(a => {
+  let t = document.querySelector("#observeme");
+  let i = 0;
+  window.onerror = function (err) {
+    t.style.height = "112px";
+    i++;
+    requestAnimationFrame(a.step_func_done(() => {
+      assert_equals(i, 1);
+    }), 0);
+  };
+
+  new ResizeObserver(function() {
+    t.style.height = "111px";
+  }).observe(observeme);
+  observeme.style.height = "110px";
+
+}, "Changing layout in window error handler should not result in lifecyle loop when resize observer loop limit is reached.");
+</script>
diff --git a/testing/web-platform/tests/resize-observer/create-pattern-data-url.js b/testing/web-platform/tests/resize-observer/create-pattern-data-url.js
new file mode 100644
index 0000000000..d3634dc424
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/create-pattern-data-url.js
@@ -0,0 +1,22 @@
+const patternSize = 4;
+
+export default function createPatternDataURL() {
+  const ctx = document.createElement('canvas').getContext('2d');
+  ctx.canvas.width = patternSize;
+  ctx.canvas.height = patternSize;
+
+  const b = [0, 0, 0, 255];
+  const t = [0, 0, 0, 0];
+  const r = [255, 0, 0, 255];
+  const g = [0, 255, 0, 255];
+
+  const imageData = new ImageData(patternSize, patternSize);
+  imageData.data.set([
+    b, t, t, r,
+    t, b, g, t,
+    t, r, b, t,
+    g, t, t, b,
+  ].flat());
+  ctx.putImageData(imageData, 0, 0);
+  return {patternSize, dataURL: ctx.canvas.toDataURL()};
+}
diff --git a/testing/web-platform/tests/resize-observer/devicepixel-ref.html b/testing/web-platform/tests/resize-observer/devicepixel-ref.html
new file mode 100644
index 0000000000..a006732015
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/devicepixel-ref.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<style>
+  #greenrect {
+    width: 100%;
+    height: 100%;
+    border: 1px solid green;
+    box-sizing: border-box;
+  }
+  #outer {
+    padding-top: 1.7px;
+    width: 300.8px;
+    height: 250.1px;
+  }
+  #outer2 {
+    padding-top: 1.4px;
+    width: 300.8px;
+    height: 250.1px;
+  }
+</style>
+<body>
+  <div id="outer">
+    <div id="greenrect"></div>
+  </div>
+  <div id="outer2">
+    <div id="greenrect"></div>
+  </div>
+</body>
diff --git a/testing/web-platform/tests/resize-observer/devicepixel.html b/testing/web-platform/tests/resize-observer/devicepixel.html
new file mode 100644
index 0000000000..e92079bea5
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/devicepixel.html
@@ -0,0 +1,85 @@
+<!doctype html>
+<link rel="match" href="devicepixel-ref.html">
+<meta name="assert" content="Device pixel content box sizes and pixel snapping are correct in Resize Observer callback">
+<!--
+  This test verifies that the device pixel content box sizes available
+  in a resize observer callback are correct. Resize observer notifications
+  are delivered as the element is loaded. A box is then drawn using the
+  available dimensions in device pixels. The result is compared to the reference
+  which uses div and border to draw a box.
+-->
+
+<style>
+  #canvas2D {
+    width: 100%;
+    height: 100%;
+  }
+  #canvas2DPadding14 {
+    width: 100%;
+    height: 100%;
+  }
+  #outer {
+    padding-top: 1.7px;
+    width: 300.8px;
+    height: 250.1px;
+  }
+  #outer2 {
+    padding-top: 1.4px;
+    width: 300.8px;
+    height: 250.1px;
+  }
+
+</style>
+<body>
+  <div id="outer">
+    <canvas id="canvas2D"></canvas>
+  </div>
+  <div id="outer2">
+    <canvas id="canvas2DPadding14"></canvas>
+  </div>
+</body>
+
+<script>
+  // Create a box using device pixel content box dimensions
+  function paint2D(ctx, snappedSize) {
+    ctx.beginPath();
+    // Use a linewidth of 2. Because the rectangle is drawn at 0,0 with
+    // its dimensions being the same as canvas dimensions, linewidth as it
+    // is drawn on the canvas will be 1.
+    ctx.lineWidth = window.devicePixelRatio * 2;
+    ctx.strokeStyle = "green";
+    ctx.rect(0, 0, snappedSize.inlineSize, snappedSize.blockSize);
+    ctx.stroke();
+  }
+
+  function updateCanvasSize2D(canvas2D, ctx, snappedSize) {
+    canvas2D.width = snappedSize.inlineSize;
+    canvas2D.height = snappedSize.blockSize;
+    paint2D(ctx, snappedSize);
+  }
+
+  (function() {
+    let canvas2D = document.querySelector("#canvas2D");
+    let canvas2DPadding14 = document.querySelector("#canvas2DPadding14");
+
+    function observeSizes() {
+      let ro = new ResizeObserver( entries => {
+        for (entry of entries) {
+          let size = entry.devicePixelContentBoxSize[0];
+          if (entry.target == canvas2D) {
+            let canvas2D = document.querySelector("#canvas2D");
+            let ctx = canvas2D.getContext("2d");
+            updateCanvasSize2D(canvas2D, ctx, size);
+          } else if (entry.target == canvas2DPadding14){
+            let canvas2DPadding14 = document.querySelector("#canvas2DPadding14");
+            let ctx = canvas2DPadding14.getContext("2d");
+            updateCanvasSize2D(canvas2DPadding14, ctx, size);
+          }
+        }
+      });
+      ro.observe(canvas2D, {box: "device-pixel-content-box"});
+      ro.observe(canvas2DPadding14, {box: "device-pixel-content-box"});
+    }
+    observeSizes();
+  })();
+</script>
diff --git a/testing/web-platform/tests/resize-observer/devicepixel2-ref.html b/testing/web-platform/tests/resize-observer/devicepixel2-ref.html
new file mode 100644
index 0000000000..7674eb0ab8
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/devicepixel2-ref.html
@@ -0,0 +1,21 @@
+<!doctype html>
+<body>
+<script type="module">
+import createPatternDataURL from './create-pattern-data-url.js';
+
+const { patternSize, dataURL } = createPatternDataURL();
+document.body.style.backgroundImage = `url("${dataURL}")`;
+
+function setBackgroundPatternTo1DevicePixel() {
+  const oneDevicePixel = 1 / devicePixelRatio;
+  const patternPixels = oneDevicePixel * patternSize;
+  document.body.style.backgroundSize = `${patternPixels}px ${patternPixels}px`;
+}
+setBackgroundPatternTo1DevicePixel();
+
+// If we're viewed interactively and the user activates
+// full-page-zoom, changes the page zoom level, or resizes
+// the window, update the rendering to account for that:
+window.addEventListener('resize', setBackgroundPatternTo1DevicePixel);
+</script>
+</body>
diff --git a/testing/web-platform/tests/resize-observer/devicepixel2.html b/testing/web-platform/tests/resize-observer/devicepixel2.html
new file mode 100644
index 0000000000..97b6255457
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/devicepixel2.html
@@ -0,0 +1,131 @@
+<!doctype html>
+<link rel="match" href="devicepixel2-ref.html">
+<meta name="assert" content="Resize Observer's reported device pixel content box size should be consistent with the actual pixel-snapped painted box size">
+<style>
+  html, body {
+    margin: 0;
+  }
+  .line {
+    width: 100px;
+    display: flex;
+  }
+  .line>* {
+    flex: 1;
+    height: 20px;
+  }
+</style>
+<body>
+  <div></div>
+  <script type="module">
+import createPatternDataURL from './create-pattern-data-url.js';
+
+const {patternSize, dataURL} = createPatternDataURL();
+
+/**
+ * Set the pattern's size on this element so that it draws where
+ * 1 pixel in the pattern maps to 1 devicePixel and then set
+ * its position so it's aligned to the pattern of the body element.
+ */
+function setPattern(elem, x, y) {
+  const oneDevicePixel = 1 / devicePixelRatio;
+  const patternPixels = oneDevicePixel * patternSize;
+  elem.style.backgroundImage = `url("${dataURL}")`;
+  elem.style.backgroundSize = `${patternPixels}px ${patternPixels}px`;
+  elem.style.backgroundPosition = `${-x * oneDevicePixel}px ${-y * oneDevicePixel}px`;
+}
+
+/*
+This test creates elements like this
+
+  <body>
+    <div>
+      <div class="line"><div></div><div></div></div>
+      <div class="line"><div></div><div></div><div></div></div>
+      <div class="line"><div></div><div></div><div></div><div></div></div>
+      ...
+    </div>
+  </body>
+
+It has the first `<div>` starting at the top left corner of the page, the
+same as the body so they should both have a devicePixel position of 0,0
+
+The devicePixelContentBoxSize of all the elements is queried with
+a ResizeObserver.
+
+It then sets a repeating background-image, background-size and
+background-position on the all of the innermost children elements so
+they have a 4x4 checkerboard image at a size that should draw each pixel
+of the 4x4 checkerboard pattern in single devicePixels and so they should
+all align with the same pattern applied in a similar manner to the body.
+
+In other words
+
+  <div class="line">
+    <div></div><div></div><div></div>
+  </div>
+
+The first child will be displayed at left = 0 so its background-position is
+set to 0px 0px. The second child should be displayed at
+
+secondChildLeft = entry(firstChild).devicePixelContentBoxSize[0].inlineSize.
+
+The 3rd child should be displayed at
+
+thirdChildLeft = entry(firstChild).devicePixelContentBoxSize[0].inlineSize +
+                 entry(secondChild).devicePixelContentBoxSize[0].inlineSize
+
+Using the children's device-pixel positions (determined as described above),
+we then set each child's background-position to the negated version of the
+position that we computed for that child. This effectively makes its pattern's
+origin match the document origin.
+
+If what devicePixelContentBox reports is correct the children's patterns
+will align with the pattern in the body and the children will be invisible.
+If what devicePixelContentBox reports is incorrect the patterns will not
+align and it should be clearly visible.
+*/
+
+const wrapperElem = document.querySelector('div');
+
+const elemToDevicePixelSize = new Map();
+
+function setPatternsUsingSizeInfo(entries) {
+  setPattern(document.body, 0, 0);
+
+  for (const entry of entries) {
+    elemToDevicePixelSize.set(entry.target, {
+      inlineSize: entry.devicePixelContentBoxSize[0].inlineSize,
+      blockSize: entry.devicePixelContentBoxSize[0].blockSize,
+    });
+  }
+
+  let devicePixelY = 0;
+  for (const lineElem of wrapperElem.children) {
+    let devicePixelX = 0;
+    for (const childElem of lineElem.children) {
+      setPattern(childElem, devicePixelX, devicePixelY);
+      const devicePixelSize = elemToDevicePixelSize.get(childElem);
+      devicePixelX += devicePixelSize.inlineSize;
+    }
+
+    const lineEntry = elemToDevicePixelSize.get(lineElem);
+    const lineHeight = lineEntry.blockSize;
+    devicePixelY += lineHeight;
+  }
+}
+
+const observer = new ResizeObserver(setPatternsUsingSizeInfo);
+observer.observe(document.body);
+for (let numFlexItems = 2; numFlexItems < 15; ++numFlexItems) {
+  const lineElem = document.createElement('div');
+  lineElem.className = 'line';
+  observer.observe(lineElem, {box:"device-pixel-content-box"});
+  for (let j = 0; j < numFlexItems; ++j) {
+    const inner = document.createElement('div');
+    lineElem.appendChild(inner);
+    observer.observe(inner, {box:"device-pixel-content-box"});
+  }
+  wrapperElem.appendChild(lineElem);
+}
+  </script>
+</body>
diff --git a/testing/web-platform/tests/resize-observer/eventloop.html b/testing/web-platform/tests/resize-observer/eventloop.html
new file mode 100644
index 0000000000..3a9e453faf
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/eventloop.html
@@ -0,0 +1,253 @@
+<!doctype html>
+<title>ResizeObserver notification event loop tests</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/resizeTestHelper.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+
+// allow uncaught exception because ResizeObserver posts exceptions
+// to window error handler when limit is exceeded.
+// This codepath is tested in this file.
+
+setup({allow_uncaught_exception: true});
+
+function template() {
+  let helper = new ResizeTestHelper(
+    "test0: title",
+  [
+    {
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+var onErrorCalled = false;
+
+window.onerror = err => {
+  onErrorCalled = true;
+}
+
+function test0() {
+  let t1 = createAndAppendElement("div");
+  let t2 = createAndAppendElement("div", t1);
+  let t3 = createAndAppendElement("div", t2);
+
+  let divs = [t1, t2, t3];
+  let rAF = 0;
+  let helper = new ResizeTestHelper(
+    "test0: multiple notifications inside same event loop",
+  [
+    {
+      setup: observer => {
+        onErrorCalled = false;
+        observer.observe(t1);
+        observer.observe(t2);
+        observer.observe(t3);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 3, "3 notifications");
+      }
+    },
+    {
+      setup: observer => {
+        helper.startCountingRaf();
+        divs.forEach( el => { el.style.width = "101px";});
+      },
+      notify: (entries, observer) => {
+        // t1 is not delivered
+        assert_equals(entries.length, 2, "2 notifications");
+        assert_equals(helper.rafCount, 0, "still in same loop");
+      }
+    },
+    {
+      setup: observer => {
+        divs.forEach( el => { el.style.width = "102px";});
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "1 notifications");
+        assert_equals(helper.rafCount, 0, "same loop");
+      }
+    },
+    { // t1 and t2 get notified
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 2, "2 notifications");
+        assert_equals(helper.rafCount, 1, "new loop");
+        assert_equals(onErrorCalled, true, "error was fired");
+        observer.disconnect();
+      }
+    }
+  ]);
+
+  return new Promise((resolve, reject) => {
+    // This test uses requestAnimationFrame() to check the count of event loop,
+    // but on some browsers, the FrameRequestCallback may be throttled (i.e.
+    // simply fired after some extra time) in cases where this test is running
+    // in an iframe that hasn't yet been painted (i.e. we're not visible).
+    // This may result in some intermittent failures if this test didn't get a
+    // first paint (and hence may not have started firing FrameRequestCallbacks)
+    // by the time the test starts expecting helper.rafCount to have changed.
+    //
+    // Therefore, we don't start the test logic until body.onload has fired.
+    // This increases the likelihood that this testcase will have gotten a
+    // chance to paint when we start invoking requestAnimationFrame, and that
+    // its rAF callbacks will fire when the test logic expects them to.
+    document.body.onload = () => resolve();
+  }).then(() => {
+    return helper.start(() => t1.remove());
+  });
+}
+
+function test1() {
+  let t1 = createAndAppendElement("div");
+  t1.style.width = '100px';
+  let t2 = createAndAppendElement("div", t1);
+  let t3 = createAndAppendElement("div", t2);
+  let shadow = t3.attachShadow({ mode: "open" });
+  let t4 = createAndAppendElement("div", shadow);
+  let t5 = createAndAppendElement("div", t4);
+
+  let resizers = [t1, t2, t3, t4, t5];
+
+  // Testing depths of shadow roots
+  // DOM: t1 <- t2 <- t3 <-shadow- t4 <- t5
+  let helper = new ResizeTestHelper(
+    "test1: depths of shadow roots",
+  [
+    {
+      setup: observer => {
+        onErrorCalled = false;
+        resizers.forEach( el => observer.observe(el) );
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 5, "all entries resized");
+      }
+    },
+    {
+      setup: observer => {
+        resizers.forEach( el => el.style.width = "111px" );
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 4, "depth limited");
+      }
+    },
+    {
+      setup: observer => {
+        resizers.forEach( el => el.style.width = "112px" );
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 3, "depth limited");
+      }
+    },
+    {
+      setup: observer => {
+        resizers.forEach( el => el.style.width = "113px" );
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 2, "depth limited");
+      }
+    },
+    {
+      setup: observer => {
+        resizers.forEach( el => el.style.width = "114px" );
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "depth limited");
+      }
+    },
+    {
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 4, "limit notifications");
+        assert_equals(onErrorCalled, true, "breached limit");
+        observer.disconnect();
+      }
+    },
+  ]);
+  return helper.start(() => t1.remove());
+}
+
+function test2() {
+  // <div id="container">
+  //   <div id="a1" style="width:100px;height:100px">
+  //     <div id="a2" style="width:100px;height:100px"></div>
+  //   </div>
+  //   <div id="b1" style="width:100px;height:100px">
+  //     <div id="b2" style="width:100px;height:100px"></div>
+  //   </div>
+  // </div>
+  let container = createAndAppendElement("div");
+  let a1 = createAndAppendElement("div", container);
+  let a2 = createAndAppendElement("div", a1);
+  let b1 = createAndAppendElement("div", container);
+  let b2 = createAndAppendElement("div", b1);
+  let targets = [a1, a2, b1, b2];
+
+  let helper = new ResizeTestHelper(
+    "test2: move target in dom while inside event loop",
+  [
+    {
+      setup: observer => {
+        for (let t of targets)
+          observer.observe(t);
+      },
+      notify: (entries, observer) => {
+        return true;  // delay next observation
+      }
+    },
+    { // resize them all
+      setup: observer => {
+        for (let t of targets)
+          t.style.width = "110px";
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, targets.length, "all targets observed");
+      }
+    },
+    { // resize all, move dom upwards
+      setup: observer => {
+        for (let t of targets)
+          t.style.width = "130px";
+        container.appendChild(b2);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "b2 moved upwards");
+        assert_equals(entries[0].target, a2);
+      }
+    },
+    { // resize all, move dom downwards
+      setup: observer => {
+        for (let t of targets)
+          t.style.width = "130px";
+        a2.appendChild(b2);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "b2 moved downwards");
+        assert_equals(entries[0].target, b2);
+      }
+    },
+  ]);
+  return helper.start(() => container.remove());
+}
+
+let guard;
+test(_ => {
+  assert_own_property(window, "ResizeObserver");
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => test1())
+  .then(() => test2())
+  .then(() => guard.done());
+
+</script>
diff --git a/testing/web-platform/tests/resize-observer/fragments.html b/testing/web-platform/tests/resize-observer/fragments.html
new file mode 100644
index 0000000000..bba94db88e
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/fragments.html
@@ -0,0 +1,115 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>ResizeObserver with multiple fragments</title>
+<link rel="author" title="Oriol Brufau" href="mailto:obrufau@igalia.com">
+<link rel="help" href="https://drafts.csswg.org/resize-observer-1/">
+<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/3673">
+<meta name="assert" content="Tests ResizeObserver supports multiple fragments." />
+
+<style>
+#wrapper {
+  column-width: 100px;
+  width: max-content;
+  height: 100px;
+  margin: 10px;
+}
+#target {
+  outline: solid;
+  background: orange;
+}
+.w50 {
+  width: 50px;
+}
+.w75 {
+  width: 75px;
+}
+.h100 {
+  height: 100px;
+}
+.h150 {
+  height: 150px;
+}
+.h175 {
+  height: 175px;
+}
+</style>
+
+<div id="log"></div>
+
+<div id="wrapper">
+  <div id="target"></div>
+</div>
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+const target = document.getElementById("target");
+
+const nextSizes = (() => {
+  let callback = null;
+  new ResizeObserver((entries) => {
+    if (callback) {
+      callback(entries[0].contentBoxSize);
+      callback = null;
+    }
+  }).observe(target);
+  return () => {
+    if (callback) {
+      throw "Already awaiting another notification";
+    }
+    return new Promise((resolve, reject) => {
+      callback = resolve;
+      requestAnimationFrame(() => {
+        requestAnimationFrame(() => {
+          reject("Missing ResizeObserver notification");
+          callback = null;
+        });
+      });
+    });
+  };
+})();
+
+function checkSizes(className, expectedSizes, msg) {
+  promise_test(async () => {
+    await new Promise(requestAnimationFrame);
+    target.className = className;
+    let sizes;
+    try {
+      sizes = await nextSizes();
+    } catch (error) {
+      assert_unreached(error);
+    }
+    assert_equals(sizes.length, expectedSizes.length, "number of fragments");
+    for (let i = 0; i < sizes.length; ++i) {
+      assert_equals(sizes[i].inlineSize, expectedSizes[i][0], `fragment #${i+1} inline size`);
+      assert_equals(sizes[i].blockSize, expectedSizes[i][1], `fragment #${i+1} block size`);
+    }
+  }, msg);
+}
+
+checkSizes(
+  "w50 h100",
+  [[50, 100]],
+  "Single fragment"
+);
+checkSizes(
+  "w50 h150",
+  [[50, 100], [50, 50]],
+  "Adding 2nd fragment"
+);
+checkSizes(
+  "w50 h175",
+  [[50, 100], [50, 75]],
+  "Resizing 2nd fragment"
+);
+checkSizes(
+  "w75 h175",
+  [[75, 100], [75, 75]],
+  "Resizing all fragments"
+);
+checkSizes(
+  "w75 h100",
+  [[75, 100]],
+  "Removing 2nd fragment"
+);
+</script>
diff --git a/testing/web-platform/tests/resize-observer/idlharness.window.js b/testing/web-platform/tests/resize-observer/idlharness.window.js
new file mode 100644
index 0000000000..2d459b0b12
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/idlharness.window.js
@@ -0,0 +1,37 @@
+// META: script=/resources/WebIDLParser.js
+// META: script=/resources/idlharness.js
+// META: script=resources/resizeTestHelper.js
+
+'use strict';
+
+// https://wicg.github.io/ResizeObserver/
+
+idl_test(
+  ['resize-observer'],
+  ['dom', 'geometry'],
+  async idl_array => {
+    idl_array.add_objects({
+      ResizeObserver: ['observer'],
+      ResizeObserverEntry: ['entry'],
+    });
+
+    const div = document.createElement('div');
+    document.body.appendChild(div);
+    let helper = new ResizeTestHelper(
+      "ResizeObserverEntry creator",
+      [
+        {
+          setup: observer => {
+            self.observer = observer;
+            observer.observe(div);
+            div.style.width = "5px";
+          },
+          notify: entries => {
+            self.entry = entries[0];
+            assert_equals(entries[0].contentRect.width, 5, "target width");
+          }
+        }
+      ]);
+    await helper.start();
+  }
+);
diff --git a/testing/web-platform/tests/resize-observer/iframe-same-origin-ref.html b/testing/web-platform/tests/resize-observer/iframe-same-origin-ref.html
new file mode 100644
index 0000000000..142db741ef
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/iframe-same-origin-ref.html
@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<title>Resize Observer: reference for the usage of iframes in the same origin</title>
+<iframe style="width: 100px; height: 100px;"
+  srcdoc="<div style='background: green; height: 50px; width: 50px;'></div">
+</iframe>
+<br>
+Observer callbacks: 3
diff --git a/testing/web-platform/tests/resize-observer/iframe-same-origin.html b/testing/web-platform/tests/resize-observer/iframe-same-origin.html
new file mode 100644
index 0000000000..22d8399b23
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/iframe-same-origin.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>Resize Observer: observed elements and ResizeObserver object are in the
+  differnt documents</title>
+<link rel="match" href="iframe-same-origin-ref.html">
+<meta name="assert" content="The resize observer callback should be notified
+  when the observed element inside an sub document while the resize observer
+  is registed in the outer document">
+
+<script src="/common/reftest-wait.js"></script>
+
+<body>
+  <iframe id="container" style="width: 100px; height: 100px;"
+    srcdoc="<div style='background: green; height: 30px; width: 50px;'></div">
+  </iframe>
+  <br>
+  Observer callbacks: <span id="callbackReport">0</span>
+</body>
+
+<script>
+  function load() {
+    return new Promise(resolve => {
+      container.onload = resolve;
+    });
+  }
+
+  let target;
+  let resolvePromise;
+  load().then(() => {
+    // Get target after loaded.
+    target = container.contentWindow.document.body.firstElementChild;
+
+    let observerCallbacks = 0;
+    const resizeObserver = new ResizeObserver(entries => {
+      callbackReport.innerText = ++observerCallbacks;
+      resolvePromise();
+    });
+    return new Promise(resolve => {
+      resolvePromise = resolve;
+      resizeObserver.observe(target);
+      // |observerCallbacks| will be increased by one here because we need to
+      // trigger notification in the event loop that contains ResizeObserver
+      // observe() call even when resize/reflow does not happen.
+    });
+  }).then(() => {
+    return new Promise(resolve => {
+      // Use requestAnimationFrame() to make sure we handle the callback in
+      // the following event loop. (This makes sure we schedule the
+      // ResizeObserver event properly for the following event loop after
+      // handling the previous one.)
+      window.requestAnimationFrame(() => {
+        resolvePromise = resolve;
+        target.style.height = "40px";
+        target.offsetHeight; // force to reflow the iframe document.
+        // |observerCallbacks| is 2 now.
+      });
+    });
+  }).then(() => {
+    return new Promise(resolve => {
+      window.requestAnimationFrame(() => {
+        resolvePromise = resolve;
+        target.style.height = "50px";
+        target.offsetHeight; // force to reflow the iframe document.
+        // |observerCallbacks| is 3 now.
+      });
+    });
+  }).then(() => {
+    // This is needed to workaround a Chromium test infrastructure bug.
+    // See https://crbug.com/1270820#c8 for details.
+    return new Promise((resolve) => window.requestAnimationFrame(resolve));
+  }).then(() => {
+    document.body.offsetHeight; // force to reflow the outer document.
+    takeScreenshot();
+  });
+</script>
+</html>
diff --git a/testing/web-platform/tests/resize-observer/notify.html b/testing/web-platform/tests/resize-observer/notify.html
new file mode 100644
index 0000000000..dc2e268b8a
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/notify.html
@@ -0,0 +1,408 @@
+<!doctype html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/resizeTestHelper.js"></script>
+<style>
+  div {
+    border: 1px dotted gray
+  }
+  .transform {
+    transform: scale(2,2) rotate(90deg)
+  }
+</style>
+<p>ResizeObserver tests</p>
+<div id="target1" style="width:100px;height:100px;">t1
+  <div id="target2" style="width:100px;height:100px;">t2
+    <div id="target3" style="width:100px;height:100px;">t3
+      <span id="inline">inline</span>
+    </div>
+  </div>
+</div>
+<div id="absolute" style="width:100.5px;height:100.5px;position:absolute;top:10.3px;left:10.3px"></div>
+<script>
+'use strict';
+
+let t1 = document.querySelector('#target1');
+let t2 = document.querySelector('#target2');
+let t3 = document.querySelector('#target3');
+let abs = document.querySelector('#absolute');
+let inline = document.querySelector('#inline');
+
+function test0() {
+  let helper = new ResizeTestHelper(
+    "test0: notification ordering",
+  [
+    {
+      setup: observer => {
+        observer.observe(t3);
+        observer.observe(t2);
+        observer.observe(t1);
+        t1.style.width = "5px";
+        t3.style.width = "5px";
+        t2.style.width = "5px";
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 3, "3 resizes");
+        assert_equals(entries[0].target, t3, "ordering");
+        assert_equals(entries[1].target, t2, "ordering");
+        assert_equals(entries[2].target, t1, "ordering");
+        observer.disconnect();
+        t1.style.width = "100px";
+        t2.style.width = "100px";
+        t3.style.width = "100px";
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test1() {
+  let helper = new ResizeTestHelper(
+    "test1: display:none triggers notification",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        t1.style.display = "none";
+      },
+      notify: (entries, observer) => {
+        t1.style.display = "";
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+
+function test2() {
+  let helper = new ResizeTestHelper(
+    "test2: remove/appendChild trigger notification",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    { // "removeChild triggers notification"
+      setup: observer => {
+        t1.parentNode.removeChild(t1);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries[0].target, t1);
+        return true;  // Delay next step
+      }
+    },
+    { // "appendChild triggers notification",
+      setup: observer => {
+        document.body.appendChild(t1);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries[0].target, t1)
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+
+function test3() {
+  let helper = new ResizeTestHelper(
+    "test3: dimensions match",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+        t1.style.width = "200.5px";
+        t1.style.height = "100px";
+        t1.style.paddingLeft = "20px";
+        t1.style.paddingTop = "10px";
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries[0].contentRect.left,20);
+        assert_equals(entries[0].contentRect.top,10);
+        assert_between_inclusive(entries[0].contentRect.width, 200.4, 200.6, "width is not rounded");
+        assert_equals(entries[0].contentRect.height, 100);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test4() {
+  let helper = new ResizeTestHelper(
+    "test4: transform do not cause notifications",
+  [
+    {
+      setup: observer => {
+        observer.observe(t2);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        t2.classList.add("transform");
+      },
+      notify: (entries, observer) => {
+        assert_unreached("transform must not trigger notifications");
+      },
+      timeout: () => {
+        t2.classList.remove("transform");
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test5() {
+  let helper = new ResizeTestHelper(
+    "test5: moving an element does not trigger notifications",
+  [
+    {
+      setup: observer => {
+        observer.observe(abs);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        abs.style.top = "20.33px";
+        abs.style.left = "20.33px";
+      },
+      notify: (entries, observer) => {
+        assert_unreached("movement should not cause resize notifications");
+      },
+      timeout: () => {
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test6() {
+  let helper = new ResizeTestHelper(
+    "test6: inline element notifies once with 0x0.",
+  [
+    {
+      setup: observer => {
+        observer.observe(inline);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "observing inline element triggers notification");
+        assert_equals(entries[0].target, inline, "observing inline element triggers notification");
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        inline.style.width = "66px";
+      },
+      notify: (entries, observer) => {
+        assert_unreached("resizing inline element should not cause resize notifications");
+      },
+      timeout: () => {
+        // expected
+      }
+    },
+    { // "inline element that becomes block should notify",
+      setup: observer => {
+        inline.style.display = "block";
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "inline element becoming a non-zero sized block triggers a notification");
+        assert_equals(entries[0].target, inline, "inline element becoming a non-zero sized block triggers a notification");
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test7() {
+  let helper = new ResizeTestHelper(
+    "test7: unobserve inside notify callback",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+        observer.observe(t2);
+      },
+      notify: (entries, observer) => {
+        t1.style.width = "777px";
+        t2.style.width = "777px";
+        observer.unobserve(t1);
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "only t2 is observed");
+        assert_equals(entries[0].target, t2, "only t2 is observed");
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test8() {
+  let helper = new ResizeTestHelper(
+    "test8: observe inside notify callback",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+      },
+      notify: (entries, observer) => {
+        observer.observe(t2);
+        t2.style.width = "888px";
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1, "only t2 is observed");
+        assert_equals(entries[0].target, t2, "only t2 is observed");
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test9() {
+  let helper = new ResizeTestHelper(
+    "test9: disconnect inside notify callback",
+  [
+    {
+      setup: observer => {
+        observer.observe(t1);
+      },
+      notify: (entries, observer) => {
+        t1.style.width = "999px";
+        observer.disconnect();
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+      },
+      notify: (entries, observer) => {
+        assert_unreached("there should be no notifications after disconnect");
+      },
+      timeout: () => {
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test10() {
+  var parent = t1.parentNode;
+  let helper = new ResizeTestHelper(
+    "test10: element notifies when parent removed",
+  [
+    {
+      setup: observer => {
+        observer.observe(t3);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        t1.parentNode.removeChild(t1);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, t3);
+        parent.appendChild(t1);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test11() {
+  let t = createAndAppendElement("div");
+  t.style.display = "none";
+
+  let helper = new ResizeTestHelper(
+    "test11: display:none element should be notified",
+    [
+      {
+        setup: observer => {
+          observer.observe(t);
+        },
+        notify: entries => {
+          assert_equals(entries.length, 1, "1 pending notification");
+          assert_equals(entries[0].target, t, "target is t");
+          assert_equals(entries[0].contentRect.width, 0, "target width");
+          assert_equals(entries[0].contentRect.height, 0, "target height");
+        }
+      }
+    ]);
+  return helper.start(() => t.remove());
+}
+
+function test12() {
+  let t = createAndAppendElement("div");
+  t.style.width = "0px";
+
+  let helper = new ResizeTestHelper(
+    "test12: element sized 0x0 should be notified",
+    [
+      {
+        setup: observer => {
+          observer.observe(t);
+        },
+        notify: entries => {
+          assert_equals(entries.length, 1, "1 pending notification");
+          assert_equals(entries[0].target, t, "target is t");
+          assert_equals(entries[0].contentRect.width, 0, "target width");
+          assert_equals(entries[0].contentRect.height, 0, "target height");
+        }
+      }
+    ]);
+  return helper.start(() => t.remove());
+}
+
+let guard;
+test(_ => {
+  assert_own_property(window, "ResizeObserver");
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => { return test1(); })
+  .then(() => { return test2(); })
+  .then(() => { return test3(); })
+  .then(() => { return test4(); })
+  .then(() => { return test5(); })
+  .then(() => { return test6(); })
+  .then(() => { return test7(); })
+  .then(() => { return test8(); })
+  .then(() => { return test9(); })
+  .then(() => { return test10(); })
+  .then(() => { return test11(); })
+  .then(() => { return test12(); })
+  .then(() => { guard.done(); });
+
+</script>
diff --git a/testing/web-platform/tests/resize-observer/observe.html b/testing/web-platform/tests/resize-observer/observe.html
new file mode 100644
index 0000000000..f6015bef78
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/observe.html
@@ -0,0 +1,1007 @@
+<!doctype html>
+<title>ResizeObserver tests</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/resizeTestHelper.js"></script>
+<body>
+<div id="log"></div>
+
+<script>
+'use strict';
+
+function test0() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test0: simple observation",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+        t.style.width = "5px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 5, "target width");
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test1() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test1: multiple observation on same element trigger only one",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+        observer.observe(t);
+        t.style.width = "10px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test2() {
+  test(() => {
+      assert_throws_js(TypeError, _=> {
+        let ro = new ResizeObserver(() => {});
+        ro.observe({});
+      });
+    },
+    "test2: throw exception when observing non-element"
+  );
+  return Promise.resolve();
+}
+
+function test3() {
+  let t1 = createAndAppendElement("div");
+  let t2 = createAndAppendElement("div");
+  t1.style.width = "100px";
+  t2.style.width = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test3: disconnect stops all notifications", [
+    {
+      setup: observer => {
+        observer.observe(t1);
+        observer.observe(t2);
+        observer.disconnect();
+        t1.style.width = "30px";
+      },
+      notify: entries => {
+         assert_unreached("no entries should be observed");
+      },
+      timeout: () => {
+        // expected
+      }
+    }
+  ]);
+  return helper.start(() => {
+    t1.remove();
+    t2.remove();
+  });
+}
+
+function test4() {
+  let t1 = createAndAppendElement("div");
+  let t2 = createAndAppendElement("div");
+  t1.style.width = "100px";
+  t2.style.width = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test4: unobserve target stops notifications, unobserve non-observed does nothing", [
+    {
+      setup: observer => {
+        observer.observe(t1);
+        observer.observe(t2);
+        observer.unobserve(t1);
+        observer.unobserve(document.body);
+        t1.style.width = "40px";
+        t2.style.width = "40px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "only t2");
+        assert_equals(entries[0].target, t2, "t2 was observed");
+      }
+    }
+  ]);
+  return helper.start(() => {
+    t1.remove();
+    t2.remove();
+  });
+}
+
+function test5() {
+  const img = new Image();
+  img.style.width = "15px";
+  img.style.height = "15px";
+  img.src = "resources/image.png";
+
+  return img.decode().then(() => {
+    return new Promise(resolve => {
+      requestAnimationFrame(() => {
+        document.body.appendChild(img);
+        resolve();
+      });
+    });
+  }).then(() => {
+    let helper = new ResizeTestHelper("test5: observe img",[
+      {
+        setup: observer => {
+          observer.observe(img);
+        },
+        notify: entries => {
+          return true;
+        }
+      },
+      {
+        setup: observer => {
+          img.style.width = "15.5px";
+        },
+        notify: entries => {
+          assert_equals(entries.length, 1);
+          assert_equals(entries[0].contentRect.width, 15.5);
+        }
+      }
+    ]);
+    return helper.start(() => img.remove());
+  }, () => {
+    // dummy test for dumping the error message.
+    test(_ => {
+      assert_unreached("decode image failed");
+    }, "test5: observe img");
+  });
+}
+
+function test6() {
+  let iframe = createAndAppendElement("iframe");
+  iframe.width = "300px";
+  iframe.height = "100px";
+  iframe.style.display = "block";
+
+  let resolvePromise;
+  let promise = new Promise((resolve) => {
+    resolvePromise = resolve;
+  });
+  let test = async_test('test6: iframe notifications');
+  let testRequested = false;
+  test.add_cleanup(() => iframe.remove());
+
+  window.addEventListener('message', event => {
+    switch(event.data) {
+    case 'readyToTest':
+      if (!testRequested) {
+        iframe.contentWindow.postMessage('startTest', '*');
+        testRequested = true;
+      }
+    break;
+    case 'success':
+    case 'fail':
+      window.requestAnimationFrame(() => {
+        resolvePromise();
+        test.step(() => {
+          assert_equals(event.data, 'success');
+          test.done();
+        });
+      });
+    break;
+    }
+  }, false);
+
+  iframe.src = "./resources/iframe.html";
+  return new Promise(function(resolve, reject) {
+    iframe.onload = () => resolve();
+    iframe.onerror = () => reject();
+  }).then(() => {
+    return promise;
+  }).catch(error => {
+    test.step(() => {
+      assert_unreached("loading iframe is error");
+    });
+  });
+}
+
+function test7() {
+  let harnessTest = async_test("test7: callback.this");
+  let resolvePromise;
+  let ro = new ResizeObserver( function(entries, obs)  {
+    let callbackThis = this;
+    harnessTest.step(() => {
+      assert_equals(callbackThis, ro, "callback.this is ResizeObserver");
+      assert_equals(obs, ro, "2nd argument is ResizeObserver");
+      ro.disconnect();
+      // every reference to RO must be null before test completes
+      // to avoid triggering test leak-detection
+      ro = null;
+      callbackThis = null;
+      obs = null;
+      harnessTest.step_timeout( _ => {
+        harnessTest.done();
+        resolvePromise();
+      }, 0);
+     });
+    }
+  );
+
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+  harnessTest.add_cleanup(() => t.remove());
+
+  ro.observe(t);
+
+  return new Promise( (resolve, reject) => {
+    resolvePromise = resolve;
+  });
+}
+
+function test8() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+  t.style.height = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test8: simple content-box observation",
+  [
+    {
+      setup: observer => {
+
+        observer.observe(t, { box: "content-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 0, "target top padding");
+        assert_equals(entries[0].contentRect.left, 0, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 100,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.width = "90px";
+        t.style.height = "90px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 90, "target width");
+        assert_equals(entries[0].contentRect.height, 90, "target height");
+        assert_equals(entries[0].contentRect.top, 0, "target top padding");
+        assert_equals(entries[0].contentRect.left, 0, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 90,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 90,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 90,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 90,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.padding = "5px";
+      },
+      notify: entries => {
+        assert_unreached("the 'content-box' ResizeObserver shouldn't fire " +
+                         "for restyles that don't affect the content-box size");
+      },
+      timeout: () => {
+        // expected
+        // Note: the border-box size is 100px x 100px right now.
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test9() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+  t.style.height = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test9: simple content-box observation but keep border-box size unchanged",
+  [
+    {
+      setup: observer => {
+        observer.observe(t, { box: "content-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 0, "target top padding");
+        assert_equals(entries[0].contentRect.left, 0, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 100,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        // Keep the border-box size the same, and change the content-box size.
+        t.style.width = "92px";
+        t.style.height = "92px";
+        t.style.padding = "4px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 92, "target width");
+        assert_equals(entries[0].contentRect.height, 92, "target height");
+        assert_equals(entries[0].contentRect.top, 4, "target top padding");
+        assert_equals(entries[0].contentRect.left, 4, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 92,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 92,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 100,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test10() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+  t.style.height = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test10: simple border-box observation",
+  [
+    {
+      setup: observer => {
+        observer.observe(t, { box: "border-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 0, "target top padding");
+        assert_equals(entries[0].contentRect.left, 0, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 100,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.padding = "4px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 4, "target top padding");
+        assert_equals(entries[0].contentRect.left, 4, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 108,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 108,
+                      "target border-box block size");
+      }
+    },
+    {
+      setup: observer => {
+        t.style.width = "104px";
+        t.style.height = "104px";
+        t.style.padding = "2px";
+      },
+      notify: entries => {
+        assert_unreached("the 'border-box' ResizeObserver shouldn't fire " +
+                         "for restyles that don't affect the border-box size");
+      },
+      timeout: () => {
+        // expected: 104 + 2 * 2 = 108. The border-box size is the same.
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test11() {
+  let wrapper = createAndAppendElement("div");
+  wrapper.style.width = "100px";
+  wrapper.style.height = "100px";
+  wrapper.style.writingMode = "vertical-rl";
+  let t = createAndAppendElement("div", wrapper);
+  t.style.inlineSize = "50px";
+  t.style.blockSize = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test11: simple observation with vertical writing mode",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 50, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 50,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 50,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.blockSize = "75px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 75, "target width");
+        assert_equals(entries[0].contentRect.height, 50, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 75,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 75,
+                      "target border-box block size");
+      }
+    }
+  ]);
+
+  return helper.start(() => {
+    t.remove();
+    wrapper.remove();
+  });
+}
+
+function test12() {
+  let t = createAndAppendElement("div");
+  t.style.writingMode = "vertical-lr";
+  t.style.inlineSize = "100px";
+  t.style.blockSize = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test12: no observation is fired after the change of writing mode when " +
+    "box's specified size comes from logical size properties.",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 50,
+                      "target content-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.writingMode = "horizontal-tb";
+      },
+      notify: entries => {
+        assert_unreached("the logical size of content-box doesn't change");
+      },
+      timeout: () => {
+        // expected: We don't change the logical size of content-box.
+      }
+    }
+  ]);
+
+  return helper.start(() => t.remove());
+}
+
+function test13() {
+  let t = createAndAppendElement("div");
+  t.style.writingMode = "vertical-lr";
+  t.style.height = "100px";
+  t.style.width = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test13: an observation is fired after the change of writing mode when " +
+    "box's specified size comes from physical size properties.",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 50,
+                      "target content-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.writingMode = "horizontal-tb";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+      },
+    }
+  ]);
+
+  return helper.start(() => t.remove());
+}
+
+function test14() {
+  let t = createAndAppendElement("div");
+  t.style.width = "100px";
+  t.style.height = "100px";
+
+  let helper = new ResizeTestHelper(
+    "test14: observe the same target but using a different box should " +
+    "override the previous one",
+  [
+    {
+      setup: observer => {
+        observer.observe(t, { box: "content-box" });
+        observer.observe(t, { box: "border-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 0, "target top padding");
+        assert_equals(entries[0].contentRect.left, 0, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 100,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        // Change border-box size.
+        t.style.padding = "4px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 100, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentRect.top, 4, "target top padding");
+        assert_equals(entries[0].contentRect.left, 4, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 100,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 108,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 108,
+                      "target border-box block size");
+      }
+    },
+    {
+      setup: observer => {
+        // Change only content-box size.
+        t.style.width = "104px";
+        t.style.height = "104px";
+        t.style.padding = "2px";
+      },
+      notify: entries => {
+        assert_unreached("the 'border-box' ResizeObserver shouldn't fire " +
+                         "for restyles that don't affect the border-box size");
+      },
+      timeout: () => {
+        // expected: 104 + 2 * 2 = 108. The border-box size is the same.
+      }
+    }
+  ]);
+  return helper.start(() => t.remove());
+}
+
+function test15() {
+  let t = createAndAppendElement("div");
+  t.style.height = "100px";
+  t.style.width = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test15: an observation is fired with box dimensions 0 when element's " +
+    "display property is set to inline",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        t.style.display = "inline";
+      },
+      notify: entries => {
+        assert_equals(entries[0].contentRect.width, 0, "target width");
+        assert_equals(entries[0].contentRect.height, 0, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 0,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 0,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 0,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 0,
+                      "target border-box block size");
+      }
+    }
+  ]);
+
+  return helper.start(() => t.remove());
+}
+
+function test16() {
+  let t = createAndAppendElement("span");
+
+  let helper = new ResizeTestHelper(
+    // See: https://drafts.csswg.org/resize-observer/#intro.
+    "test16: observations fire once with 0x0 size for non-replaced inline elements",
+  [
+    {
+      setup: observer => {
+        observer.observe(t);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 0, "target width");
+        assert_equals(entries[0].contentRect.height, 0, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 0,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 0,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 0,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 0,
+                      "target border-box block size");
+      }
+    }
+  ]);
+
+  return helper.start(() => t.remove());
+
+}
+
+function test17() {
+  // <div id="outer">
+  //   <div id="nested">
+  //   </div>
+  // </div>
+
+  let outer = document.createElement('div');
+  outer.style.width = "100px";
+  outer.style.height = "100px";
+  outer.style.padding = "10px";
+  outer.style.border = "1px solid blue"
+  let nested = document.createElement('div');
+  nested.style.width = "60px";
+  nested.style.height = "50px";
+  nested.style.padding = "5%";
+  nested.style.boxSizing = "border-box";
+  nested.style.border = "5px solid black";
+  outer.appendChild(nested);
+  document.body.appendChild(outer);
+
+  let helper = new ResizeTestHelper(
+    "test17: Box sizing snd Resize Observer notifications",
+  [
+    {
+      setup: observer => {
+        observer.observe(nested, { box: "content-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, nested, "target is nested");
+        assert_equals(entries[0].contentRect.width, 40, "target width");
+        assert_equals(entries[0].contentRect.height, 30, "target height");
+        assert_equals(entries[0].contentRect.top, 5, "target top padding");
+        assert_equals(entries[0].contentRect.left, 5, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 40,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 30,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 60,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 50,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      // Changes to a parent's dimensions with a child's padding set as a percentage
+      // should fire observation if content-box is being observed
+      setup: observer => {
+        outer.style.height = "200px";
+        outer.style.width = "200px";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, nested, "target is nested");
+        assert_equals(entries[0].contentRect.width, 30, "target width");
+        assert_equals(entries[0].contentRect.height, 20, "target height");
+        assert_equals(entries[0].contentRect.top, 10, "target top padding");
+        assert_equals(entries[0].contentRect.left, 10, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 30,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 20,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 60,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 50,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      // Changes to a parent's dimensions with a child's padding set as a percentage
+      // should fire observation if content-box is being observed
+      setup: observer => {
+        nested.style.border = "1px solid black";
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, nested, "target is nested");
+        assert_equals(entries[0].contentRect.width, 38, "target width");
+        assert_equals(entries[0].contentRect.height, 28, "target height");
+        assert_equals(entries[0].contentRect.top, 10, "target top padding");
+        assert_equals(entries[0].contentRect.left, 10, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 38,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 28,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 60,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 50,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        observer.observe(nested, { box: "border-box" });
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, nested, "target is nested");
+        assert_equals(entries[0].contentRect.width, 38, "target width");
+        assert_equals(entries[0].contentRect.height, 28, "target height");
+        assert_equals(entries[0].contentRect.top, 10, "target top padding");
+        assert_equals(entries[0].contentRect.left, 10, "target left padding");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 38,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 28,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 60,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 50,
+                      "target border-box block size");
+        return true;
+      }
+    },
+    {
+      // Changes to a parent's dimensions with a child's padding set as a percentage
+      // should not fire observation if border-box is being observed
+      setup: observer => {
+        outer.style.height = "100px";
+      },
+      notify: entries => {
+        assert_unreached("No observation should be fired when nested border box remains constant");
+      },
+      timeout: () => {
+        // expected
+      }
+    },
+
+  ]);
+  return helper.start(() => nested.remove());
+}
+
+function test18() {
+  let t = createAndAppendElement("div");
+  t.style.height = "100px";
+  t.style.width = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test18: an observation is fired when device-pixel-content-box is being " +
+    "observed",
+  [
+    {
+      setup: observer => {
+        observer.observe(t, {box: "device-pixel-content-box"});
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 50,
+                      "target device-pixel-content-box inline size");
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 100,
+                      "target device-pixel-content-box block size");
+      }
+    },
+  ]);
+
+  return helper.start(() => t.remove());
+}
+
+function test19() {
+  // zoom is not a standard css property, so we should check it first. If the
+  // browser doesn't support it, we skip this test.
+  if (!CSS.supports("zoom", "3")) {
+    return Promise.resolve();
+  }
+
+  let t = createAndAppendElement("div");
+  t.style.height = "100px";
+  t.style.width = "50px";
+
+  let helper = new ResizeTestHelper(
+    "test19: an observation is fired when device-pixel-content-box is being " +
+    "observed and zoom change",
+  [
+    {
+      setup: observer => {
+        observer.observe(t, {box: "device-pixel-content-box"});
+      },
+      notify: entries => {
+        // No need to test again (see test18), so skip this event loop.
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        document.body.style.zoom = 3;
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1, "1 pending notification");
+        assert_equals(entries[0].target, t, "target is t");
+        assert_equals(entries[0].contentRect.width, 50, "target width");
+        assert_equals(entries[0].contentRect.height, 100, "target height");
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50,
+                      "target content-box inline size");
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 100,
+                      "target content-box block size");
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50,
+                      "target border-box inline size");
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 100,
+                      "target border-box block size");
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 150,
+                      "target device-pixel-content-box inline size");
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 300,
+                      "target device-pixel-content-box block size");
+        return true;
+      }
+    },
+    {
+      setup: observer => {
+        document.body.style.zoom = '';
+      },
+      notify: entries => {}
+    }
+  ]);
+
+  return helper.start(() => t.remove());
+}
+
+let guard;
+test(_ => {
+  assert_own_property(window, "ResizeObserver");
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => test1())
+  .then(() => test2())
+  .then(() => test3())
+  .then(() => test4())
+  .then(() => test5())
+  .then(() => test6())
+  .then(() => test7())
+  .then(() => test8())
+  .then(() => test9())
+  .then(() => test10())
+  .then(() => test11())
+  .then(() => test12())
+  .then(() => test13())
+  .then(() => test14())
+  .then(() => test15())
+  .then(() => test16())
+  .then(() => test17())
+  .then(() => test18())
+  .then(() => test19())
+  .then(() => guard.done());
+
+</script>
diff --git a/testing/web-platform/tests/resize-observer/ordering.html b/testing/web-platform/tests/resize-observer/ordering.html
new file mode 100644
index 0000000000..1cd9950c53
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/ordering.html
@@ -0,0 +1,24 @@
+<!doctype html>
+<title>ResizeObserver and IntersectionObserver ordering</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+  async_test(function(t) {
+    let sawResize = false;
+    let sawIo = false;
+    let resizeObserver = new ResizeObserver(t.step_func(function() {
+      assert_false(sawIo, "ResizeObserver notification should be delivered before IntersectionObserver notification");
+      sawResize = true;
+      resizeObserver.disconnect();
+    }));
+
+    let io = new IntersectionObserver(t.step_func_done(function() {
+      assert_true(sawResize, "IntersectionObserver notification should be delivered after ResizeObserver notification");
+      sawIo = true;
+      io.disconnect();
+    }));
+
+    resizeObserver.observe(document.documentElement);
+    io.observe(document.documentElement);
+  });
+</script>
diff --git a/testing/web-platform/tests/resize-observer/resources/iframe.html b/testing/web-platform/tests/resize-observer/resources/iframe.html
new file mode 100644
index 0000000000..5c801e6368
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/resources/iframe.html
@@ -0,0 +1,38 @@
+<!doctype html>
+<head>
+  <script src="./resizeTestHelper.js"></script>
+</head>
+<p>iframe test</p>
+<div id="itarget1" style="width:100px;height:100px;">t1</div>
+<script>
+'use strict';
+let t1 = document.querySelector('#itarget1');
+function test0() {
+  let timeoutId = window.setTimeout( () => {
+    window.parent.postMessage('fail', '*');
+  }, ResizeTestHelper.TIMEOUT);
+  let ro = new ResizeObserver(function(entries) {
+    window.clearTimeout(timeoutId);
+    window.parent.postMessage('success', '*');
+  });
+  ro.observe(t1);
+}
+let testStarted = false;
+window.addEventListener('message', function(ev) {
+    switch(ev.data) {
+        case 'startTest':
+          testStarted = true;
+          test0();
+        break;
+    }
+});
+// How does parent know we've loaded problem is solved by
+// broadcasting readyToTest message repeatedly until test starts.
+function broadcastReady() {
+  if (!testStarted) {
+    window.parent.postMessage('readyToTest', '*');
+    window.requestAnimationFrame(broadcastReady);
+  }
+}
+window.onload = broadcastReady;
+</script>
diff --git a/testing/web-platform/tests/resize-observer/resources/image.png b/testing/web-platform/tests/resize-observer/resources/image.png
new file mode 100644
index 0000000000..51741584a0
Binary files /dev/null and b/testing/web-platform/tests/resize-observer/resources/image.png differ
diff --git a/testing/web-platform/tests/resize-observer/resources/resizeTestHelper.js b/testing/web-platform/tests/resize-observer/resources/resizeTestHelper.js
new file mode 100644
index 0000000000..284a780c25
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/resources/resizeTestHelper.js
@@ -0,0 +1,195 @@
+'use strict';
+
+/**
+  ResizeTestHelper is a framework to test ResizeObserver
+  notifications. Use it to make assertions about ResizeObserverEntries.
+  This framework is needed because ResizeObservations are
+  delivered asynchronously inside the event loop.
+
+  Features:
+  - can queue multiple notification steps in a test
+  - handles timeouts
+  - returns Promise that is fulfilled when test completes.
+    Use to chain tests (since parallel async ResizeObserver tests
+    would conflict if reusing same DOM elements).
+
+  Usage:
+
+  create ResizeTestHelper for every test.
+  Make assertions inside notify, timeout callbacks.
+  Start tests with helper.start()
+  Chain tests with Promises.
+  Counts animation frames, see startCountingRaf
+*/
+
+/*
+  @param name: test name
+  @param steps:
+  {
+    setup: function(ResizeObserver) {
+      // called at the beginning of the test step
+      // your observe/resize code goes here
+    },
+    notify: function(entries, observer) {
+      // ResizeObserver callback.
+      // Make assertions here.
+      // Return true if next step should start on the next event loop.
+    },
+    timeout: function() {
+      // Define this if your test expects to time out, and the expected timeout
+      // value will be 100ms.
+      // If undefined, timeout is assert_unreached after 1000ms.
+    }
+  }
+*/
+function ResizeTestHelper(name, steps)
+{
+    this._name = name;
+    this._steps = steps || [];
+    this._stepIdx = -1;
+    this._harnessTest = null;
+    this._observer = new ResizeObserver(this._handleNotification.bind(this));
+    this._timeoutBind = this._handleTimeout.bind(this);
+    this._nextStepBind = this._nextStep.bind(this);
+}
+
+// The default timeout value in ms.
+// We expect TIMEOUT to be longer than we would ever have to wait for notify()
+// to be fired. This is used for tests which are not expected to time out, so
+// it can be large without slowing down the test.
+ResizeTestHelper.TIMEOUT = 1000;
+// A reasonable short timeout value in ms.
+// We expect SHORT_TIMEOUT to be long enough that notify() would usually get a
+// chance to fire before SHORT_TIMEOUT expires. This is used for tests which
+// *are* expected to time out (with no callbacks fired), so we'd like to keep
+// it relatively short to avoid slowing down the test.
+ResizeTestHelper.SHORT_TIMEOUT = 100;
+
+ResizeTestHelper.prototype = {
+  get _currentStep() {
+    return this._steps[this._stepIdx];
+  },
+
+  _nextStep: function() {
+    if (++this._stepIdx == this._steps.length)
+      return this._done();
+    // Use SHORT_TIMEOUT if this step expects timeout.
+    let timeoutValue = this._steps[this._stepIdx].timeout ?
+        ResizeTestHelper.SHORT_TIMEOUT :
+        ResizeTestHelper.TIMEOUT;
+    this._timeoutId = this._harnessTest.step_timeout(
+      this._timeoutBind, timeoutValue);
+    try {
+      this._steps[this._stepIdx].setup(this._observer);
+    }
+    catch(err) {
+      this._harnessTest.step(() => {
+        assert_unreached("Caught a throw, possible syntax error");
+      });
+    }
+  },
+
+  _handleNotification: function(entries) {
+    if (this._timeoutId) {
+      window.clearTimeout(this._timeoutId);
+      delete this._timeoutId;
+    }
+    this._harnessTest.step(() => {
+      try {
+        let rafDelay = this._currentStep.notify(entries, this._observer);
+        if (rafDelay)
+          window.requestAnimationFrame(this._nextStepBind);
+        else
+          this._nextStep();
+      }
+      catch(err) {
+        this._harnessTest.step(() => {
+          throw err;
+        });
+        // Force to _done() the current test.
+        this._done();
+      }
+    });
+  },
+
+  _handleTimeout: function() {
+    delete this._timeoutId;
+    this._harnessTest.step(() => {
+      if (this._currentStep.timeout) {
+        this._currentStep.timeout();
+      }
+      else {
+        this._harnessTest.step(() => {
+          assert_unreached("Timed out waiting for notification. (" + ResizeTestHelper.TIMEOUT + "ms)");
+        });
+      }
+      this._nextStep();
+    });
+  },
+
+  _done: function() {
+    this._observer.disconnect();
+    delete this._observer;
+    this._harnessTest.done();
+    if (this._rafCountRequest) {
+      window.cancelAnimationFrame(this._rafCountRequest);
+      delete this._rafCountRequest;
+    }
+    window.requestAnimationFrame(() => { this._resolvePromise(); });
+  },
+
+  start: function(cleanup) {
+    this._harnessTest = async_test(this._name);
+
+    if (cleanup) {
+      this._harnessTest.add_cleanup(cleanup);
+    }
+
+    this._harnessTest.step(() => {
+      assert_equals(this._stepIdx, -1, "start can only be called once");
+      this._nextStep();
+    });
+    return new Promise( (resolve, reject) => {
+      this._resolvePromise = resolve;
+      this._rejectPromise = reject;
+    });
+  },
+
+  get rafCount() {
+    if (!this._rafCountRequest)
+      throw "rAF count is not active";
+    return this._rafCount;
+  },
+
+  get test() {
+    if (!this._harnessTest) {
+      throw "_harnessTest is not initialized";
+    }
+    return this._harnessTest;
+  },
+
+  _incrementRaf: function() {
+    if (this._rafCountRequest) {
+      this._rafCount++;
+      this._rafCountRequest = window.requestAnimationFrame(this._incrementRafBind);
+    }
+  },
+
+  startCountingRaf: function() {
+    if (this._rafCountRequest)
+      window.cancelAnimationFrame(this._rafCountRequest);
+    if (!this._incrementRafBind)
+      this._incrementRafBind = this._incrementRaf.bind(this);
+    this._rafCount = 0;
+    this._rafCountRequest = window.requestAnimationFrame(this._incrementRafBind);
+  }
+}
+
+function createAndAppendElement(tagName, parent) {
+  if (!parent) {
+    parent = document.body;
+  }
+  const element = document.createElement(tagName);
+  parent.appendChild(element);
+  return element;
+}
diff --git a/testing/web-platform/tests/resize-observer/scrollbars-2.html b/testing/web-platform/tests/resize-observer/scrollbars-2.html
new file mode 100644
index 0000000000..51b470c8a2
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/scrollbars-2.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<title>ResizeObserver content-box size and scrollbars</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1733042">
+<style>
+  #scrollContainer {
+    width: 100px;
+    height: 100px;
+    /* Should be bigger than any reasonable scrollbar */
+    padding: 30px;
+    border: 10px solid blue;
+    overflow: scroll;
+    background: #818182;
+  }
+
+</style>
+<div id="scrollContainer"></div>
+<script>
+  promise_test(async function() {
+    let count = 0;
+
+    const scrollContainer = document.getElementById('scrollContainer');
+    // 20 to account for the border.
+    const scrollbarSize = scrollContainer.offsetWidth - scrollContainer.clientWidth - 20;
+    let size = await new Promise(resolve => {
+      const observer = new ResizeObserver(entries => {
+        resolve(entries[0].contentBoxSize[0]);
+      });
+      observer.observe(scrollContainer);
+    });
+
+    assert_equals(size.inlineSize, 100 - scrollbarSize);
+    assert_equals(size.blockSize, 100 - scrollbarSize);
+  });
+</script>
diff --git a/testing/web-platform/tests/resize-observer/scrollbars.html b/testing/web-platform/tests/resize-observer/scrollbars.html
new file mode 100644
index 0000000000..129e74e5cd
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/scrollbars.html
@@ -0,0 +1,56 @@
+<!DOCTYPE html>
+<title>ResizeObserver content-box size and scrollbars</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1733042">
+<style>
+  #outer {
+    position: relative;
+    width: 100px;
+    height: 200px;
+    overflow: auto;
+    background: #818182;
+  }
+
+  #inner {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 10px;
+    height: 10px;
+    background: #0a6fc0;
+  }
+</style>
+<div id="outer">
+  <div id="inner"></div>
+</div>
+<script>
+  async function animationFrame() {
+    return new Promise(r => requestAnimationFrame(r));
+  }
+
+  // This test is expected to fail with overlay scrollbars.
+  promise_test(async function() {
+    let count = 0;
+
+    const outer = document.getElementById('outer');
+    const inner = document.getElementById('inner');
+    const observer = new ResizeObserver(entries => {
+      count++;
+    });
+
+    observer.observe(outer);
+
+    inner.style.top = '1000px';
+
+    await animationFrame();
+    await animationFrame();
+
+    inner.style.top = 0;
+
+    await animationFrame();
+    await animationFrame();
+
+    assert_equals(count, 2, "ResizeObserver should subtract scrollbar sizes from content-box rect");
+  });
+</script>
diff --git a/testing/web-platform/tests/resize-observer/svg-with-css-box-001.html b/testing/web-platform/tests/resize-observer/svg-with-css-box-001.html
new file mode 100644
index 0000000000..4ec0a7de72
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/svg-with-css-box-001.html
@@ -0,0 +1,91 @@
+<!doctype html>
+<title>ResizeObserver for SVG elements with CSS box.</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/resizeTestHelper.js"></script>
+<div id="container" style="width: 500px; height: 500px;">
+<svg id="svg" width="100%" viewBox="0 0 100 100">
+  <circle cx="50" cy="50" r="45" style="fill:orange;stroke:black;stroke-width:1" />
+  <foreignObject id="foreign" x="0" y="0" width="100" height="100">
+    <svg xmlns="http://www.w3.org/2000/svg"
+         width="100%" height="100%"
+         viewBox="0 0 100 100"
+         id="foreign-svg">
+      <circle cx="50" cy="50" r="45" style="fill:orange;stroke:black;stroke-width:1" />
+    </svg>
+  </foreignObject>
+</svg>
+<script>
+'use strict';
+
+function test0() {
+  let targetWidth = 150;
+  let target = document.getElementById('foreign-svg');
+  let container = document.getElementById('foreign');
+  let helper = new ResizeTestHelper(
+    "test0: observe `foreignObject` SVG in HTML document",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', targetWidth);
+      },
+      notify: entries => {
+        assert_equals(entries.length, 1);
+        const entry = entries[0];
+        assert_equals(entry.target, target);
+        assert_equals(entry.contentBoxSize[0].inlineSize, targetWidth);
+      },
+    }
+  ]);
+  return helper.start();
+}
+
+function test1() {
+  let targetWidth = 400;
+  let target = document.getElementById('svg');
+  let container = document.getElementById('container');
+  let helper = new ResizeTestHelper(
+    "test1: observe inline SVG in HTML",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.style.width = targetWidth + 'px';
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        const entry = entries[0];
+        assert_equals(entry.target, target);
+        assert_equals(entry.contentBoxSize[0].inlineSize, targetWidth);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+let guard;
+test(_ => {
+  assert_implements(window.ResizeObserver);
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => { test1(); })
+  .then(() => { guard.done(); });
+
+</script>
diff --git a/testing/web-platform/tests/resize-observer/svg-with-css-box-002.svg b/testing/web-platform/tests/resize-observer/svg-with-css-box-002.svg
new file mode 100644
index 0000000000..3c009641db
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/svg-with-css-box-002.svg
@@ -0,0 +1,94 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg"
+     xmlns:h="http://www.w3.org/1999/xhtml"
+     version="1.1"
+     width="100%" height="100%"
+     id="root">
+<title>ResizeObserver for SVG elements with CSS box.</title>
+<h:script src="/resources/testharness.js"/>
+<h:script src="/resources/testharnessreport.js"/>
+<h:script src="./resources/resizeTestHelper.js"/>
+
+<foreignObject x="0" y="0" width="100" height="100">
+  <svg xmlns="http://www.w3.org/2000/svg"
+       width="100" height="100"
+       viewBox="0 0 100 100"
+       id="foreign-svg">
+    <circle cx="50" cy="50" r="45" style="fill:orange;stroke:black;stroke-width:1" />
+  </svg>
+</foreignObject>
+
+<script>
+'use strict';
+
+function test0() {
+  let targetWidth = 400;
+  let target = document.getElementById('root');
+  let helper = new ResizeTestHelper(
+    "test0: Root SVG resize observed",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', targetWidth);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        const entry = entries[0];
+        assert_equals(entry.target, target);
+        assert_equals(entry.contentBoxSize[0].inlineSize, targetWidth);
+      }
+    }
+  ]);
+  return helper.start(() => {
+    target.setAttribute('width', '100%');
+  });
+}
+
+function test1() {
+  let targetWidth = 90;
+  let target = document.getElementById('foreign-svg');
+  let helper = new ResizeTestHelper(
+    "test1: `foreignObject` SVG resize observed",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true; // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', targetWidth);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        const entry = entries[0];
+        assert_equals(entry.target, target);
+        assert_equals(entry.contentBoxSize[0].inlineSize, targetWidth);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+let guard;
+test(_ => {
+  assert_implements(window.ResizeObserver);
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => { test1(); })
+  .then(() => { guard.done(); });
+</script>
+</svg>
diff --git a/testing/web-platform/tests/resize-observer/svg.html b/testing/web-platform/tests/resize-observer/svg.html
new file mode 100644
index 0000000000..6511afc8b0
--- /dev/null
+++ b/testing/web-platform/tests/resize-observer/svg.html
@@ -0,0 +1,619 @@
+<!doctype html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="./resources/resizeTestHelper.js"></script>
+<p>ResizeObserver svg tests</p>
+<svg height="430" width="500" >
+  <circle cx="10" cy="10" r="5" style="fill:orange;stroke:black;stroke-width:1" />
+  <ellipse cx="10" cy="30" rx="5" ry="5" style="fill:orange;stroke:black;stroke-width:1"/>
+  <foreignObject cy="50" width="100" height="20">
+    <body>
+      <p>Here is a paragraph that requires word wrap</p>
+    </body>
+  </foreignObject>
+  <image xlink:href="" x="0" y="100" height="30" width="100" />
+  <line x1="0" y1="50" x2="20" y2="70" stroke="black" stroke-width="2"/>
+  <path d="M 0 100 L 100 100 L 50 150 z"
+        style="fill:orange;stroke:black;stroke-width:1" />
+  <polygon points="0,200 100,200 50,250" style="fill:orange;stroke:black;stroke-width:1" />
+  <polyline points="0,300 100,300 50,350" style="fill:orange;stroke:black;stroke-width:1"/>
+  <rect x="0" y="380" width="10" height="10" style="fill:orange; stroke:black; stroke-width:1" />
+  <text x="0" y="400" font-size="20" font-family="Ahem">svg text tag</text>
+  <g fill="white" stroke="green" stroke-width="5">
+    <rect x="0" y="380" width="50" height="20" id="g_rect" />
+  </g>
+</svg>
+<script>
+'use strict';
+
+setup({allow_uncaught_exception: true});
+
+function test0() {
+  let target = document.querySelector('circle');
+  let helper = new ResizeTestHelper(
+    "test0: observe svg:circle",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('r', 10);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test1() {
+  let target = document.querySelector('ellipse');
+  let helper = new ResizeTestHelper(
+    "test1: observe svg:ellipse",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('rx', 10);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 20);
+        assert_equals(entries[0].contentRect.height, 10);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test2() {
+  let target = document.querySelector('foreignObject');
+  let helper = new ResizeTestHelper(
+    "test2: observe svg:foreignObject",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', 200);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 200);
+        assert_equals(entries[0].contentRect.height, 20);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test3() {
+  let target = document.querySelector('image');
+  let helper = new ResizeTestHelper(
+    "test3: observe svg:image",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('height', 40);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 100);
+        assert_equals(entries[0].contentRect.height, 40);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test4() {
+  let target = document.querySelector('line');
+  let helper = new ResizeTestHelper(
+    "test4: observe svg:line",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('y2', 80);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 20);
+        assert_equals(entries[0].contentRect.height, 30);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test5() {
+  let target = document.querySelector('path');
+  let helper = new ResizeTestHelper(
+    "test5: observe svg:path",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('d', "M 0 100 L 100 100 L 50 160 z");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 100);
+        assert_equals(entries[0].contentRect.height, 60);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test6() {
+  let target = document.querySelector('polygon');
+  let helper = new ResizeTestHelper(
+    "test6: observe svg:polygon",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('points', "0,200 100,200 50,260");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 100);
+        assert_equals(entries[0].contentRect.height, 60);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test7() {
+  let target = document.querySelector('polyline');
+  let helper = new ResizeTestHelper(
+    "test7: observe svg:polyline",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('points', "0,300 100,300 50,360");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 100);
+        assert_equals(entries[0].contentRect.height, 60);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test8() {
+  let target = document.querySelector('rect');
+  let helper = new ResizeTestHelper(
+    "test8: observe svg:rect",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', "20");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 20);
+        assert_equals(entries[0].contentRect.height, 10);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test9() {
+  let target = document.querySelector('text');
+  let helper = new ResizeTestHelper(
+    "test9: observe svg:text",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('font-size', "25");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+
+function test10() {
+  let target = document.querySelector('svg');
+  let helper = new ResizeTestHelper(
+    "test10: observe svg:svg, top/left is 0 even with padding",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.top, 0);
+        assert_equals(entries[0].contentRect.left, 0);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test11() {
+  // <svg>
+  //   <view></view>
+  //   <defs>
+  //     <linearGradient>
+  //       <stop></stop>
+  //     </linearGradient>
+  //   </defs>
+  // </svg>
+  const svgNS = "http://www.w3.org/2000/svg";
+  let svg = document.createElementNS(svgNS, 'svg');
+  document.body.appendChild(svg);
+
+  let view = document.createElementNS(svgNS, 'view');
+  svg.appendChild(view);
+
+  let defs = document.createElementNS(svgNS, 'defs');
+  let linearGradient = document.createElementNS(svgNS, 'linearGradient');
+  let stop = document.createElementNS(svgNS, 'stop');
+  linearGradient.appendChild(stop);
+  defs.appendChild(linearGradient);
+  svg.appendChild(defs);
+
+  let helper = new ResizeTestHelper(
+    "test11: observe svg non-displayable element",
+  [
+    {
+      setup: observer => {
+        observer.observe(view);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, view);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
+      }
+    },
+    {
+      setup: observer => {
+        observer.observe(stop);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].target, stop);
+        assert_equals(entries[0].contentRect.width, 0);
+        assert_equals(entries[0].contentRect.height, 0);
+      }
+    },
+  ]);
+  return helper.start(() => svg.remove());
+}
+
+function test12() {
+  let target = document.querySelector('rect');
+  let helper = new ResizeTestHelper(
+    "test12: observe svg:rect content box",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', 45);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 45);
+        assert_equals(entries[0].contentRect.height, 10);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 45);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 10);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test13() {
+  let target = document.querySelector('rect');
+  let helper = new ResizeTestHelper(
+    "test13: observe svg:rect border box",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', 20);
+        target.setAttribute('height', 20);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 20);
+        assert_equals(entries[0].contentRect.height, 20);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 20);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 20);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 20);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 20);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test14() {
+  let target = document.querySelector('#g_rect');
+  let helper = new ResizeTestHelper(
+    "test14: observe g:rect content and border box",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 50);
+        assert_equals(entries[0].contentRect.height, 20);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 20);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 20);
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('width', 15);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 15);
+        assert_equals(entries[0].contentRect.height, 20);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 15);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 20);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 15);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 20);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test15() {
+  let target = document.querySelector('text');
+  let helper = new ResizeTestHelper(
+    "test15: observe svg:text content and border box",
+  [
+    {
+      setup: observer => {
+        observer.observe(target);
+      },
+      notify: (entries, observer) => {
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('font-size', "30");
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 360);
+        assert_equals(entries[0].contentRect.height, 30);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 360);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 360);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 30);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test16() {
+  let target = document.querySelector('#g_rect');
+  let helper = new ResizeTestHelper(
+    "test16: observe g:rect content, border and device-pixel-content boxes",
+  [
+    {
+      setup: observer => {
+        observer.observe(target, {box: "device-pixel-content-box"});
+        target.setAttribute('width', 50);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 50);
+        assert_equals(entries[0].contentRect.height, 20);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 20);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 20);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 20);
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        target.setAttribute('height', 30);
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 50);
+        assert_equals(entries[0].contentRect.height, 30);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 30);
+      }
+    }
+  ]);
+  return helper.start();
+}
+
+function test17() {
+  // zoom is not a standard css property, so we should check it first. If the
+  // browser doesn't support it, we skip this test.
+  if (!CSS.supports("zoom", "0.1")) {
+    return Promise.resolve();
+  }
+
+  let target = document.querySelector('#g_rect');
+  let helper = new ResizeTestHelper(
+    "test17: observe g:rect content, border and device-pixel-content boxes with zoom",
+  [
+    {
+      setup: observer => {
+        observer.observe(target, {box: "device-pixel-content-box"});
+        target.setAttribute('width', 50);
+        target.setAttribute('height', 30);
+        document.body.style.zoom = 0.1;
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 50);
+        assert_equals(entries[0].contentRect.height, 30);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 5);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 3);
+        return true;  // Delay next step
+      }
+    },
+    {
+      setup: observer => {
+        document.body.style.zoom = 10;
+      },
+      notify: (entries, observer) => {
+        assert_equals(entries.length, 1);
+        assert_equals(entries[0].contentRect.width, 50);
+        assert_equals(entries[0].contentRect.height, 30);
+        assert_equals(entries[0].contentBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].contentBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].borderBoxSize[0].inlineSize, 50);
+        assert_equals(entries[0].borderBoxSize[0].blockSize, 30);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].inlineSize, 500);
+        assert_equals(entries[0].devicePixelContentBoxSize[0].blockSize, 300);
+      },
+    }
+  ]);
+  return helper.start();
+}
+
+let guard;
+test(_ => {
+  assert_own_property(window, "ResizeObserver");
+  guard = async_test('guard');
+}, "ResizeObserver implemented")
+
+test0()
+  .then(() => { return test1(); })
+  .then(() => { return test2(); })
+  .then(() => { return test3(); })
+  .then(() => { return test4(); })
+  .then(() => { return test5(); })
+  .then(() => { return test6(); })
+  .then(() => { return test7(); })
+  .then(() => { return test8(); })
+  .then(() => { return test9(); })
+  .then(() => { return test10(); })
+  .then(() => { return test11(); })
+  .then(() => { return test12(); })
+  .then(() => { return test13(); })
+  .then(() => { return test14(); })
+  .then(() => { return test15(); })
+  .then(() => { return test16(); })
+  .then(() => { return test17(); })
+  .then(() => { guard.done(); });
+
+</script>
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