[xsl] support for XSLT media type in XML stylesheet processing instruction

[Julian Reschke <julian.reschke@xxxxxx>]

Hi,

historically, UAs have only supported "text/xsl", which never was 
registered as media type for XSLT.

For now, IE9 seems to be the only shipping UA that understand the 
registered media type -- application/xslt+xml -- as well.

In Firefox, this was just fixed, and might ship with Firefox 7 (*) (see 
<https://bugzilla.mozilla.org/show_bug.cgi?id=231395>).

For Chrome / Safari / Opera I don't have any information about whether 
there are plans to add support -- feedback appreciated.

Best regards, Julian

(*) Sounds scary, but actually means "end of year".