Error Message Feedback: Unknown Parse Mode!

Validating

Hi,
I just used the validator for my site 
http://www.starwars-web.de/vuffi-raa/index.htm . As I expected, there 
weren't any problems and the page was declared valid. But one thing was 
new, though.
Above the message:

"This Page Is Valid -//W3C//DTD HTML 4.01 Strict//EN!"

there was a paragraph:

"Unknown Parse Mode!
The MIME Media Type (|text/html|) for this document is used to serve 
both SGML and XML based documents, and it is not possible to 
disambiguate it based on the |DOCTYPE| Declaration in your document. 
Parsing will continue in SGML mode."

I don't think that is a serious problem, since the validator used SGML 
mode automatically, but nevertheless I'm interested how I have to change 
my |DOCTYPE| Declaration that it is no longer impossible to disambiguate 
the mode.
So I hope it doesn't require to much effort to explain to me what I have 
to do about that.

Yours sincerely,

Marcus Weber