<!DOCTYPE html> <html class="reftest-wait"> <html> <head> <style type="text/css"> #last-child { height: 20px; margin-bottom: 0px; background-color: blue; } #separator { height: 20px; background-color: green; } #parent { margin-bottom: 20px; } </style> <script type="text/javascript"> function test() { document.getElementById('last-child').style.marginBottom = '-30px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test); </script> </head> <body> <div id="parent"> <div id="last-child"></div> </div> <div id="separator"></div> </body> </html>