<html class="reftest-wait"> <head> <style> div { transition: opacity 0.1s; } </style> <script> function showItem() { var el = document.getElementById("two"); getComputedStyle(el).opacity; // flush styles el.style.opacity = 0.99; document.addEventListener("transitionend", function() { document.documentElement.removeAttribute("class"); }, false); } window.addEventListener("MozReftestInvalidate", showItem, false); </script> </head> <body> <div id="one" style="width:50px; height:50px; background-color:blue; position:absolute"></div> <div id="two" style="width:500px; height:500px; background-color:red; opacity:0.5"></div> </body> </html>