Mediawiki does not use HTTP1.1 Status code. The lisft is here :
Mediawiki can use that to help some users.
When the page doesn't exist, Mediwiki returns code 200 : OK. When a special page
is not found, Mediawiki returns code 200, and when the page contains a redirect,
it's 200 too.
It is not very good.
And it's easy to add this feature ...
A simple php function can do it !
We have to define error code to return. There are the choice !
(a 404 error code can be return when a special page isn't found)
Scuse me foor my poor english :(