<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> .separator { height: 20px; background-color: green; } #parent { margin-top: 10px; margin-bottom: 30px; } #child { margin-top: 20px; } </style> <script type="text/javascript"> function test() { document.getElementById('child').style.marginBottom = '40px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test, false); </script> </head> <body> <div class="separator"></div> <div id="parent"> <div id="child"></div> </div> <div class="separator"></div> </body> </html>