I'm using OW 5.11 and Mac OS X 10.6.8. I just went to
www.flattr.com and the page loaded with no problems in just a couple of seconds and as far as I can tell from the source code that site is still on HTML 4.
It may be that you have a problem elsewhere or maybe your Java or other software needs updating.
Also try doing a Repair in Disk Utility as it might be that something needs fixing there.