encoding: not always required

Hello,

I have a document encoded in ASCII (a subset of UTF-8).
The XML 1.0 specification states:

It is also a fatal error if an XML entity contains no encoding declaration 
and its content is not legal UTF-8 or UTF-16.

However, the validator should therefore validate correctly XHTML documents
starting with <?xml version="1.0"?> followed by a proper XHTML 1.0 DTD
followed by the actual content.

However the validator.w3.org program insisted that I ought to specify it,
but that's not what the XML standard says, right?

Thanks for the clarification,

Neil