<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> #parent { min-height: 100px; margin-bottom: -20px; background-color: lightgreen; } #last-child { display: none; height: 110px; margin-bottom: -20px; background-color: green; } #separator { height: 20px; background-color: blue; } </style> <script type="text/javascript"> function test() { document.getElementById('last-child').style.display = 'block'; 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>