<!DOCTYPE html> <html class="reftest-wait"> <head> <style> .flexContainer { display: flex; width: 300px; height: 300px; background: yellow; } .flexItem { border: 1px dashed purple; } </style> <script> function finish() { document.documentElement.removeAttribute('class'); } </script> </head> <body onload="setTimeout(finish, 0)"> <div class="flexContainer"> <embed src="about:blank" class="flexItem"></embed> </div> </body> </html>