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Just that Microsoft uses them should not cause us to 
violate internet standards. RFC 2045 specifies that the 
syntax of a subtype is

subtype := extension-token / iana-token
iana-token := <A publicly-defined extension token. Tokens
               of this form must be registered with IANA
               as specified in RFC 2048.>
x-token := <The two characters "X-" or "x-" followed, with
            no intervening white space, by any token>

Since text/xsl and text/xul are not registered as 
specified in RFC 2048, they are not valid types.

So our choice is to either follow Microsoft, or follow the 
Internet Standards. If the inclusion of text/xsl in the 
patch was intentional (rather than a mistake), I recommend 
to reject this patch.