diff --git a/dom/animation/test/css-animations/file_event-dispatch.html b/dom/animation/test/css-animations/file_event-dispatch.html
new file mode 100644
index 0000000000..266205bc32
--- /dev/null
+++ b/dom/animation/test/css-animations/file_event-dispatch.html
@@ -0,0 +1,252 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Tests for CSS animation event dispatch</title>
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#event-dispatch"/>
+<script src="../testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { margin-left: 0px; }
+    to { margin-left: 100px; }
+  }
+</style>
+<body>
+<script>
+'use strict';
+
+/**
+ * Helper class to record the elapsedTime member of each event.
+ * The EventWatcher class in testharness.js allows us to wait on
+ * multiple events in a certain order but only records the event
+ * parameters of the most recent event.
+ */
+function AnimationEventHandler(target) {
+  this.target = target;
+  this.target.onanimationstart = function(evt) {
+   this.animationstart = evt.elapsedTime;
+  }.bind(this);
+  this.target.onanimationiteration = function(evt) {
+    this.animationiteration = evt.elapsedTime;
+  }.bind(this);
+  this.target.onanimationend = function(evt) {
+    this.animationend = evt.elapsedTime;
+  }.bind(this);
+}
+AnimationEventHandler.prototype.clear = function() {
+  this.animationstart = undefined;
+  this.animationiteration = undefined;
+  this.animationend = undefined;
+}
+
+function setupAnimation(t, animationStyle) {
+  var div = addDiv(t, { style: "animation: " + animationStyle });
+  var watcher = new EventWatcher(t, div, [ 'animationstart',
+                                           'animationiteration',
+                                           'animationend' ]);
+  var handler = new AnimationEventHandler(div);
+  var animation = div.getAnimations()[0];
+
+  return [animation, watcher, handler, div];
+}
+
+promise_test(function(t) {
+  // Add 1ms delay to ensure that the delay is not included in the elapsedTime.
+  const [animation, watcher] = setupAnimation(t, 'anim 100s 1ms');
+
+  return watcher.wait_for('animationstart').then(function(evt) {
+    assert_equals(evt.elapsedTime, 0.0);
+  });
+}, 'Idle -> Active');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] = setupAnimation(t, 'anim 100s');
+
+  // Seek to After phase.
+  animation.finish();
+  return watcher.wait_for([ 'animationstart',
+                            'animationend' ]).then(function() {
+    assert_equals(handler.animationstart, 0.0);
+    assert_equals(handler.animationend, 100);
+  });
+}, 'Idle -> After');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] =
+    setupAnimation(t, 'anim 100s 100s paused');
+
+  return animation.ready.then(function() {
+    // Seek to Active phase.
+    animation.currentTime = 100 * MS_PER_SEC;
+    return watcher.wait_for('animationstart');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 0.0);
+  });
+}, 'Before -> Active');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] =
+    setupAnimation(t, 'anim 100s 100s paused');
+
+  return animation.ready.then(function() {
+    // Seek to After phase.
+    animation.finish();
+    return watcher.wait_for([ 'animationstart', 'animationend' ]);
+  }).then(function(evt) {
+    assert_equals(handler.animationstart, 0.0);
+    assert_equals(handler.animationend, 100.0);
+  });
+}, 'Before -> After');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] =
+    setupAnimation(t, 'anim 100s 100s paused');
+
+  // Seek to Active phase.
+  animation.currentTime = 100 * MS_PER_SEC;
+  return watcher.wait_for('animationstart').then(function() {
+    // Seek to Before phase.
+    animation.currentTime = 0;
+    return watcher.wait_for('animationend');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 0.0);
+  });
+}, 'Active -> Before');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] = setupAnimation(t, 'anim 100s paused');
+
+  return watcher.wait_for('animationstart').then(function(evt) {
+    // Seek to After phase.
+    animation.finish();
+    return watcher.wait_for('animationend');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 100.0);
+  });
+}, 'Active -> After');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] =
+    setupAnimation(t, 'anim 100s 100s paused');
+
+  // Seek to After phase.
+  animation.finish();
+  return watcher.wait_for([ 'animationstart',
+                            'animationend' ]).then(function() {
+    // Seek to Before phase.
+    animation.currentTime = 0;
+    handler.clear();
+    return watcher.wait_for([ 'animationstart', 'animationend' ]);
+  }).then(function() {
+    assert_equals(handler.animationstart, 100.0);
+    assert_equals(handler.animationend, 0.0);
+  });
+}, 'After -> Before');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] =
+    setupAnimation(t, 'anim 100s 100s paused');
+
+  // Seek to After phase.
+  animation.finish();
+  return watcher.wait_for([ 'animationstart',
+                            'animationend' ]).then(function() {
+    // Seek to Active phase.
+    animation.currentTime = 100 * MS_PER_SEC;
+    handler.clear();
+    return watcher.wait_for('animationstart');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 100.0);
+  });
+}, 'After -> Active');
+
+promise_test(function(t) {
+  const [animation, watcher, handler]
+    = setupAnimation(t, 'anim 100s 100s 3 paused');
+
+  return animation.ready.then(function() {
+    // Seek to iteration 0 (no animationiteration event should be dispatched)
+    animation.currentTime = 100 * MS_PER_SEC;
+    return watcher.wait_for('animationstart');
+  }).then(function(evt) {
+    // Seek to iteration 2
+    animation.currentTime = 300 * MS_PER_SEC;
+    handler.clear();
+    return watcher.wait_for('animationiteration');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 200);
+    // Seek to After phase (no animationiteration event should be dispatched)
+    animation.currentTime = 400 * MS_PER_SEC;
+    return watcher.wait_for('animationend');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 300);
+  });
+}, 'Active -> Active (forwards)');
+
+promise_test(function(t) {
+  const [animation, watcher, handler] = setupAnimation(t, 'anim 100s 100s 3');
+
+  // Seek to After phase.
+  animation.finish();
+  return watcher.wait_for([ 'animationstart',
+                            'animationend' ]).then(function() {
+    // Seek to iteration 2 (no animationiteration event should be dispatched)
+    animation.pause();
+    animation.currentTime = 300 * MS_PER_SEC;
+    return watcher.wait_for('animationstart');
+  }).then(function() {
+    // Seek to mid of iteration 0 phase.
+    animation.currentTime = 200 * MS_PER_SEC;
+    return watcher.wait_for('animationiteration');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 200.0);
+    // Seek to before phase (no animationiteration event should be dispatched)
+    animation.currentTime = 0;
+    return watcher.wait_for('animationend');
+  });
+}, 'Active -> Active (backwards)');
+
+promise_test(function(t) {
+  const [animation, watcher, handler, div] =
+    setupAnimation(t, 'anim 100s paused');
+  return watcher.wait_for('animationstart').then(function(evt) {
+    // Seek to Idle phase.
+    div.style.display = 'none';
+    flushComputedStyle(div);
+
+    // FIXME: bug 1302648: Add test for animationcancel event here.
+
+    // Restart this animation.
+    div.style.display = '';
+    return watcher.wait_for('animationstart');
+  });
+}, 'Active -> Idle -> Active: animationstart is fired by restarting animation');
+
+promise_test(function(t) {
+  const [animation, watcher, handler, div] =
+    setupAnimation(t, 'anim 100s 100s 2 paused');
+
+  // Make After.
+  animation.finish();
+  return watcher.wait_for([ 'animationstart',
+                            'animationend' ]).then(function(evt) {
+    animation.playbackRate = -1;
+    return watcher.wait_for('animationstart');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 200);
+    // Seek to 1st iteration
+    animation.currentTime = 200 * MS_PER_SEC - 1;
+    return watcher.wait_for('animationiteration');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 100);
+    // Seek to before
+    animation.currentTime = 100 * MS_PER_SEC - 1;
+    return watcher.wait_for('animationend');
+  }).then(function(evt) {
+    assert_equals(evt.elapsedTime, 0);
+    assert_equals(animation.playState, 'running'); // delay
+  });
+}, 'Negative playbackRate sanity test(Before -> Active -> Before)');
+
+done();
+</script>
+</body>
+</html>
diff --git a/dom/animation/test/css-animations/file_event-order.html b/dom/animation/test/css-animations/file_event-order.html
new file mode 100644
index 0000000000..da78b6541c
--- /dev/null
+++ b/dom/animation/test/css-animations/file_event-order.html
@@ -0,0 +1,160 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>Tests for CSS animation event order</title>
+<link rel="help" href="https://drafts.csswg.org/css-animations-2/#event-dispatch"/>
+<script src="../testcommon.js"></script>
+<style>
+  @keyframes anim {
+    from { margin-left: 0px; }
+    to { margin-left: 100px; }
+  }
+</style>
+<body>
+<script type='text/javascript'>
+'use strict';
+
+/**
+ * Asserts that the set of actual and received events match.
+ * @param actualEvents   An array of the received AnimationEvent objects.
+ * @param expectedEvents A series of array objects representing the expected
+ *        events, each having the form:
+ *          [ event type, target element, elapsed time ]
+ */
+function checkEvents(actualEvents, ...expectedEvents) {
+  assert_equals(actualEvents.length, expectedEvents.length,
+                `Number of actual events (${actualEvents.length}: \
+${actualEvents.map(event => event.type).join(', ')}) should match expected \
+events (${expectedEvents.map(event => event.type).join(', ')})`);
+
+  actualEvents.forEach((actualEvent, i) => {
+    assert_equals(expectedEvents[i][0], actualEvent.type,
+                  'Event type should match');
+    assert_equals(expectedEvents[i][1], actualEvent.target,
+                  'Event target should match');
+    assert_equals(expectedEvents[i][2], actualEvent.elapsedTime,
+                  'Event\'s elapsed time should match');
+  });
+}
+
+function setupAnimation(t, animationStyle, receiveEvents) {
+  const div = addDiv(t, { style: "animation: " + animationStyle });
+  const watcher = new EventWatcher(t, div, [ 'animationstart',
+                                             'animationiteration',
+                                             'animationend' ]);
+
+  ['start', 'iteration', 'end'].forEach(name => {
+    div['onanimation' + name] = function(evt) {
+    receiveEvents.push({ type:        evt.type,
+                         target:      evt.target,
+                         elapsedTime: evt.elapsedTime });
+    }.bind(this);
+  });
+
+  const animation = div.getAnimations()[0];
+
+  return [animation, watcher, div];
+}
+
+promise_test(function(t) {
+  let events = [];
+  const [animation1, watcher1, div1] =
+    setupAnimation(t, 'anim 100s 2 paused', events);
+  const [animation2, watcher2, div2] =
+    setupAnimation(t, 'anim 100s 2 paused', events);
+
+  return Promise.all([ watcher1.wait_for('animationstart'),
+                       watcher2.wait_for('animationstart') ]).then(function() {
+    checkEvents(events, ['animationstart', div1, 0],
+                        ['animationstart', div2, 0]);
+
+    events.length = 0;  // Clear received event array
+
+    animation1.currentTime = 100 * MS_PER_SEC;
+    animation2.currentTime = 100 * MS_PER_SEC;
+    return Promise.all([ watcher1.wait_for('animationiteration'),
+                         watcher2.wait_for('animationiteration') ]);
+  }).then(function() {
+    checkEvents(events, ['animationiteration', div1, 100],
+                        ['animationiteration', div2, 100]);
+
+    events.length = 0;  // Clear received event array
+
+    animation1.finish();
+    animation2.finish();
+
+    return Promise.all([ watcher1.wait_for('animationend'),
+                         watcher2.wait_for('animationend') ]);
+  }).then(function() {
+    checkEvents(events, ['animationend', div1, 200],
+                        ['animationend', div2, 200]);
+  });
+}, 'Test same events are ordered by elements.');
+
+promise_test(function(t) {
+  let events = [];
+  const [animation1, watcher1, div1] =
+    setupAnimation(t, 'anim 200s 400s', events);
+  const [animation2, watcher2, div2] =
+    setupAnimation(t, 'anim 300s 2', events);
+
+  return watcher2.wait_for('animationstart').then(function(evt) {
+    animation1.currentTime = 400 * MS_PER_SEC;
+    animation2.currentTime = 400 * MS_PER_SEC;
+
+    events.length = 0;  // Clear received event array
+
+    return Promise.all([ watcher1.wait_for('animationstart'),
+                         watcher2.wait_for('animationiteration') ]);
+  }).then(function() {
+    checkEvents(events, ['animationiteration', div2, 300],
+                        ['animationstart',     div1, 0]);
+  });
+}, 'Test start and iteration events are ordered by time.');
+
+promise_test(function(t) {
+  let events = [];
+  const [animation1, watcher1, div1] =
+    setupAnimation(t, 'anim 150s', events);
+  const [animation2, watcher2, div2] =
+    setupAnimation(t, 'anim 100s 2', events);
+
+  return Promise.all([ watcher1.wait_for('animationstart'),
+                       watcher2.wait_for('animationstart') ]).then(function() {
+    animation1.currentTime = 150 * MS_PER_SEC;
+    animation2.currentTime = 150 * MS_PER_SEC;
+
+    events.length = 0;  // Clear received event array
+
+    return Promise.all([ watcher1.wait_for('animationend'),
+                         watcher2.wait_for('animationiteration') ]);
+  }).then(function() {
+    checkEvents(events, ['animationiteration', div2, 100],
+                        ['animationend',       div1, 150]);
+  });
+}, 'Test iteration and end events are ordered by time.');
+
+promise_test(function(t) {
+  let events = [];
+  const [animation1, watcher1, div1] =
+    setupAnimation(t, 'anim 100s 100s', events);
+  const [animation2, watcher2, div2] =
+    setupAnimation(t, 'anim 100s 2', events);
+
+  animation1.finish();
+  animation2.finish();
+
+  return Promise.all([ watcher1.wait_for([ 'animationstart',
+                                           'animationend' ]),
+                       watcher2.wait_for([ 'animationstart',
+                                           'animationend' ]) ]).then(function() {
+    checkEvents(events, ['animationstart', div2, 0],
+                        ['animationstart', div1, 0],
+                        ['animationend',   div1, 100],
+                        ['animationend',   div2, 200]);
+  });
+}, 'Test start and end events are sorted correctly when fired simultaneously');
+
+done();
+</script>
+</body>
+</html>
diff --git a/dom/animation/test/css-animations/test_event-dispatch.html b/dom/animation/test/css-animations/test_event-dispatch.html
new file mode 100644
index 0000000000..de3be03017
--- /dev/null
+++ b/dom/animation/test/css-animations/test_event-dispatch.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<meta charset=utf-8>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+setup({explicit_done: true});
+SpecialPowers.pushPrefEnv(
+  { "set": [["dom.animations-api.core.enabled", true]]},
+  function() {
+    window.open("file_event-dispatch.html");
+  });
+</script>
+</html>
diff --git a/dom/animation/test/css-animations/test_event-order.html b/dom/animation/test/css-animations/test_event-order.html
new file mode 100644
index 0000000000..57f1f3876b
--- /dev/null
+++ b/dom/animation/test/css-animations/test_event-order.html
@@ -0,0 +1,15 @@
+<!doctype html>
+<meta charset=utf-8>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+'use strict';
+setup({explicit_done: true});
+SpecialPowers.pushPrefEnv(
+  { "set": [["dom.animations-api.core.enabled", true]]},
+  function() {
+    window.open("file_event-order.html");
+  });
+</script>
+</html>
diff --git a/dom/animation/test/mochitest.ini b/dom/animation/test/mochitest.ini
index c694e4d25a..07e04d4d61 100644
--- a/dom/animation/test/mochitest.ini
+++ b/dom/animation/test/mochitest.ini
@@ -19,6 +19,8 @@ support-files =
   css-animations/file_document-get-animations.html
   css-animations/file_effect-target.html
   css-animations/file_element-get-animations.html
+  css-animations/file_event-dispatch.html
+  css-animations/file_event-order.html
   css-animations/file_keyframeeffect-getkeyframes.html
   css-animations/file_pseudoElement-get-animations.html
   css-transitions/file_animation-cancel.html
@@ -73,6 +75,8 @@ support-files =
 [css-animations/test_document-get-animations.html]
 [css-animations/test_effect-target.html]
 [css-animations/test_element-get-animations.html]
+[css-animations/test_event-dispatch.html]
+[css-animations/test_event-order.html]
 [css-animations/test_keyframeeffect-getkeyframes.html]
 [css-animations/test_pseudoElement-get-animations.html]
 [css-transitions/test_animation-cancel.html]
diff --git a/dom/events/test/test_legacy_event.html b/dom/events/test/test_legacy_event.html
index d772be1066..b2105a6dfc 100644
--- a/dom/events/test/test_legacy_event.html
+++ b/dom/events/test/test_legacy_event.html
@@ -73,22 +73,15 @@ function triggerShortAnimation(node) {
   node.style.animation = "anim1 1ms linear";
 }
 
-// This function triggers a long animation with two iterations, which is
-// *nearly* at the end of its first iteration.  It will hit the end of that
-// iteration (firing an event) almost immediately, 1ms in the future.
+// This function triggers a very short (10ms long) animation with many
+// iterations, which will cause a start event followed by an iteration event
+// on each subsequent tick, to fire.
 //
-// NOTE: It's important that this animation have a *long* duration.  If it were
-// short (e.g. 1ms duration), then we might jump past all its iterations in
-// a single refresh-driver tick. And if that were to happens, we'd *never* fire
-// any animationiteration events -- the CSS Animations spec says this event
-// must not be fired "...when an animationend event would fire at the same time"
-// (which would be the case in this example with a 1ms duration). So, to make
-// sure our event does fire, we use a long duration and a nearly-as-long
-// negative delay. This ensures we hit the end of the first iteration right
-// away, and that we don't risk hitting the end of the second iteration at the
-// same time.
+// NOTE: We need the many iterations since if an animation frame coincides
+// with the animation starting or ending we dispatch only the start or end
+// event and not the iteration event.
 function triggerAnimationIteration(node) {
-  node.style.animation = "anim1 300s -299.999s linear 2";
+  node.style.animation = "anim1 10ms linear 20000";
 }
 
 // GENERAL UTILITY FUNCTIONS
diff --git a/layout/reftests/transform-3d/animate-backface-hidden.html b/layout/reftests/transform-3d/animate-backface-hidden.html
index ce957bf73f..27b5870061 100644
--- a/layout/reftests/transform-3d/animate-backface-hidden.html
+++ b/layout/reftests/transform-3d/animate-backface-hidden.html
@@ -17,7 +17,7 @@ body { padding: 50px }
   height: 200px; width: 200px;
   backface-visibility: hidden;
   /* use a -99.9s delay to start at 99.9% and then move to 0% */
-  animation: flip 100s -99.9s linear 2;
+  animation: flip 100s -99.9s linear 2 paused;
 }
 
 </style>
@@ -27,7 +27,13 @@ body { padding: 50px }
 
 <script>
 
-document.getElementById("test").addEventListener("animationiteration", IterationListener, false);
+document.getElementById("test").addEventListener("animationstart", StartListener, false);
+
+function StartListener(event) {
+  var test = document.getElementById("test");
+  test.style.animationPlayState = 'running';
+  test.addEventListener("animationiteration", IterationListener, false);
+}
 
 function IterationListener(event) {
   setTimeout(RemoveReftestWait, 0);
diff --git a/layout/reftests/transform-3d/animate-preserve3d-parent.html b/layout/reftests/transform-3d/animate-preserve3d-parent.html
index ae3fec1961..d05beac6ff 100644
--- a/layout/reftests/transform-3d/animate-preserve3d-parent.html
+++ b/layout/reftests/transform-3d/animate-preserve3d-parent.html
@@ -18,7 +18,7 @@ body { padding: 50px }
   border: 1px solid black;
   transform-style: preserve-3d;
   /* use a -99.9s delay to start at 99.9% and then move to 0% */
-  animation: spin 100s -99.9s linear 2;
+  animation: spin 100s -99.9s linear 2 paused;
 }
 
 #child {
@@ -39,7 +39,13 @@ body { padding: 50px }
 
 <script>
 
-document.getElementById("parent").addEventListener("animationiteration", IterationListener, false);
+document.getElementById("parent").addEventListener("animationstart", StartListener, false);
+
+function StartListener(event) {
+  var test = document.getElementById("parent");
+  test.style.animationPlayState = 'running';
+  test.addEventListener("animationiteration", IterationListener, false);
+}
 
 function IterationListener(event) {
   setTimeout(RemoveReftestWait, 0);
diff --git a/layout/style/test/test_animations.html b/layout/style/test/test_animations.html
index eaccba1221..4019af77fc 100644
--- a/layout/style/test/test_animations.html
+++ b/layout/style/test/test_animations.html
@@ -1194,9 +1194,6 @@ advance_clock(0); // complete pending animation start
 is_approx(px_to_num(cs.marginRight), 100 * gTF.ease_in(0.4), 0.01,
           "large negative delay test at 0ms");
 check_events([{ type: 'animationstart', target: div,
-                animationName: 'anim2', elapsedTime: 3.6,
-                pseudoElement: "" },
-              { type: 'animationiteration', target: div,
                 animationName: 'anim2', elapsedTime: 3.6,
                 pseudoElement: "" }],
              "right after start in large negative delay test");
diff --git a/layout/style/test/test_animations_omta.html b/layout/style/test/test_animations_omta.html
index 4b276c8965..0b2a61ecc7 100644
--- a/layout/style/test/test_animations_omta.html
+++ b/layout/style/test/test_animations_omta.html
@@ -1407,9 +1407,6 @@ addAsyncAnimTest(function *() {
   omta_is_approx("opacity", gTF.ease_in(0.4), 0.01, RunningOn.Compositor,
                  "large negative delay test at 0ms");
   check_events([{ type: 'animationstart', target: gDiv,
-                  animationName: 'anim2', elapsedTime: 3.6,
-                  pseudoElement: "" },
-                { type: 'animationiteration', target: gDiv,
                   animationName: 'anim2', elapsedTime: 3.6,
                   pseudoElement: "" }],
                "right after start in large negative delay test");
