media-type = string.

media-type specifies the media type (MIME content type) of the data that results from outputting the result tree. The charset parameter should not be specified explicitly; instead, when the top-level media type is text, a charset parameter should be added according to the character encoding actually used by the output method.

See also:
s ection 16 of the XSL Transformations (XSLT) W3C Recommendation