<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #separator { height: 40px; background-color: green; } #first-child { display: none; height: 40px; margin-top: -10px; background-color: blue; } #parent { margin-top: -20px; } </style> <script type="text/javascript"> function test() { document.getElementById('first-child').style.display = 'block'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test); </script> </head> <body> <div id="separator"></div> <div id="parent"> <div id="first-child"></div> </div> </body> </html>