<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #a { height: 20px; background-color: green; margin-bottom: 20px; } #b { height: 20px; background-color: blue; margin-top: 0; } </style> <script type="text/javascript"> function test() { document.getElementById('b').style.marginTop = '-20px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test, false); </script> </head> <body> <div id="a"></div> <div id="b"></div> </body> </html>