<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #parent { width: 200px; } #block1 { height: 40px; margin-bottom: 10%; background-color: green; } #block2 { height: 40px; background-color: green; } </style> <script type="text/javascript"> function test() { document.getElementById('block2').style.marginTop = '25%'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test, false); </script> </head> <body> <div id="parent"> <div id="block1"></div> <div id="block2"></div> </div> </body> </html>