<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> html, body { margin: 0; padding: 5px; } div { height: 20px; background-color: green; } #small { margin-top: 40px; margin-bottom: -40px; } #big { margin-top: 0; background-color: blue; } </style> <script type="text/javascript"> function test() { document.getElementById('big').style.marginTop = '-60px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test, false); </script> </head> <body> <div id="small"></div> <div id="big"></div> </body> </html>