Web Animations API: Sort keyframes by offset

LayoutTests/web-animations-api/element-animate-list-of-keyframes.html

Index: LayoutTests/web-animations-api/element-animate-list-of-keyframes.html
diff --git a/LayoutTests/web-animations-api/element-animate-list-of-keyframes.html b/LayoutTests/web-animations-api/element-animate-list-of-keyframes.html
index 35c06e8e94dbd660dfbd4eb1b061f4f6e3d0eb3b..8f3b6fed197ae5a64c614852f38932099fbfa991 100644
--- a/LayoutTests/web-animations-api/element-animate-list-of-keyframes.html
+++ b/LayoutTests/web-animations-api/element-animate-list-of-keyframes.html
@@ -50,6 +50,26 @@ test(function() {
     assert_equals(e2Style.backgroundColor, 'rgb(64, 64, 64)');
 }, 'Calling animate() should start an animation. CamelCase property names should be parsed.');
 
+test(function() {
+    var player = e1.animate([
+        {opacity: '0.75', offset: 0.5},
+        {opacity: '0.9', offset: 1},
+        {opacity: '1', offset: 0}
+    ], durationValue);
+    player.pause();
+    player.currentTime = durationValue / 4;
+    assert_equals(e1Style.opacity, '0.875');
+}, 'Keyframes with offsets should become sorted by offset.');
+
+test(function() {
+    var keyframes = [
+        {opacity: '0.75'},
+        {opacity: '0.9', offset: 1},
+        {opacity: '1', offset: 0}
+    ];
+    assert_throws('InvalidModificationError', function() { e1.animate(keyframes, durationValue); });
+}, 'Should throw when keyframes are not loosely sorted and any have no offset.');
+
 var keyframesWithInvalid = [
         {width: '0px', backgroundColor: 'octarine', offset: 0},
         {width: '1000px', foo: 'bar', offset: 1}];