Qt's DRT implements mechanisms for handling error pages in its DRT. It is a attemptive to get the DRT even more close to a real browser. See bug 30573 and *specially* https://bugs.webkit.org/show_bug.cgi?id=31509#c0 for details.
The current test** relying on this feature is Skipped in mac for now.
This bug is to track implementation in MAC's DRT.