<!DOCTYPE html> <html class="reftest-wait"> <head> <style type="text/css"> body { font-size: 20px; } #block1 { height: 20px; margin-bottom: 10em; background-color: blue; } #block2 { height: 20px; margin-top: 5em; background-color: blue; } </style> <script type="text/javascript"> function test() { document.getElementById('block2').style.fontSize = '50px'; document.documentElement.removeAttribute('class'); } document.addEventListener('MozReftestInvalidate', test); </script> </head> <body> <div id="block1"></div> <div id="block2"></div> </body> </html>