--- a/layout/style/test/test_animations_event_order.html
+++ b/layout/style/test/test_animations_event_order.html
@@ -163,16 +163,18 @@ divs.forEach((div, i) => {
   div.setAttribute('id', 'div' + i);
 });
 
 var extraStyle = document.createElement('style');
 document.head.appendChild(extraStyle);
 var sheet = extraStyle.sheet;
 sheet.insertRule('#div0::after { animation: anim 10s }', 0);
 sheet.insertRule('#div0::before { animation: anim 10s }', 1);
+sheet.insertRule('#div0::after, #div0::before { ' +
+                 ' content: " " }', 2);
 getComputedStyle(divs[0]).animationName; // build animation
 getComputedStyle(divs[1]).animationName; // build animation
 
 advance_clock(0);
 checkEventOrder([ divs[0], 'animationstart' ],
                 [ divs[0], '::before', 'animationstart' ],
                 [ divs[0], '::after', 'animationstart' ],
                 [ divs[1], 'animationstart' ],