It seems that IE8′s standards compliance may not be nearly as impressive as we had hoped. As reported on Download Squad and elsewhere, IE8 only passes the Acid2 standards compliance test if you visit the official one: if you visit an identical mirror elsewhere on the web, it will fail. Microsoft’s reasoning for this?
IE8 fails the copies of ACID2 due to the cross domain security checks IE performs for ActiveX controls. Since IE does not natively handle HTML content in the OBJECT tag, but rather uses IE’s rendering engine as an ActiveX to display this HTML content, the same cross domain security checks also apply.
While it seems like a legitimate enough reason to fail, and Microsoft is understandably scared stiff about anything that might even vaguely resemble a security hole these days, I can’t help but wonder why the other browsers don’t have this problem.