Improving XML error pages (YSOD)

Hi all,

  I thought I might have a look at bug 418305 (Make Yellow-Screen-Of-
Death More Informative). My first effort was going to be to make the
error page look a bit more like the new network error pages.

  Currently the YSOD page is produced in nsXMLContentSink which
creates a page with a "<parsererror>" root element. This is relied on
by other code and extensions so I guess it should stay the same.

  I tried adding a processing instruction into the parsererror
document, transforming it into a error page with XSLT, but I got an
error saying that the stylesheet couldn't be loaded.

  Then I thought I might be able to use an nsIContentSniffer to detect
the "<parsererror>", but the content sniffer sees the original XML
page, not the error page.

  Any ideas on better ways to do this? I could just enhance
nsXMLContentSink to produce a more complex page, but it might be
better to leave that interface only for backwards-compatibility.

Matthew