<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #parent { background-color: green; } #first-child { height: 20px; } #last-child { margin-top: 60px; } #separator { height: 20px; margin-top: 10px; background-color: green; } </style> <script type="text/javascript"> function test() { document.getElementById('last-child').style.marginBottom = '-20px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test); </script> </head> <body> <div id="parent"> <div id="first-child"></div> <div id="last-child"></div> </div> <div id="separator"></div> </body> </html>