<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #separator { height: 20px; background-color: green; } #parent { background-color: green; margin-top: 10px; } #first-child { margin-top: 20px; } #last-child { height: 20px; margin-top: 15px; } </style> <script type="text/javascript"> function test() { document.getElementById('first-child').style.marginBottom = '30px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test, false); </script> </head> <body> <div id="separator"></div> <div id="parent"> <div id="first-child"></div> <div id="last-child"></div> </div> </body> </html>