You can make a new rendering system backwards compatable without also shipping the old system in with it - and having a piece of code, or MetaData that tells Internet Explorer to use the old Renderer.

It would make sense - but it again would be an inconvenience. I'm sure they'll just switch to the standards. Not that much looks different in Firefox; and it won't be too hard for people to move to XHTML. Everybody else has to.