<!DOCTYPE HTML> <html class="reftest-wait"> <head> <style type="text/css"> #rear { width: 500px; height: 1500px; box-shadow: 0 0 71px #667; display: block; } .cover { position: absolute; width: 520px; height: 100px; background: yellow; } </style> <script> function doTest() { var es = document.getElementsByClassName("cover"); for (var i = 0; i < es.length; ++i) { es[i].style.display = 'none'; } document.documentElement.removeAttribute("class"); } window.addEventListener("MozReftestInvalidate", doTest, false); </script> </head> <body> <div id="rear"></div> <div class="cover" style="top:100px"></div> <div class="cover" style="top:300px"></div> </body> </html>