Hi!
Nein, das wäre kein wirklich gültiges XHTML-Dokument mehr.
Doch: http://www.w3.org/TR/xhtml1/#strict:
» An XML declaration is not required in all XML documents; however XHTML document authors are strongly encouraged to use XML declarations in all their documents. Such a declaration is required when the character encoding of the document is other than the default UTF-8 or UTF-16 and no encoding was determined by a higher-level protocol. «
Gruß Herbalizer