org-html-xml-declaration

org-html-xml-declaration is a variable defined in `ox-html.el'.
Its value is

(("html" . "")
 ("php" . "\"; ?>"))

Documentation:
The extension for exported HTML files.
%s will be replaced with the charset of the exported file.
This may be a string, or an alist with export extensions
and corresponding declarations.

This declaration only applies when exporting to XHTML.

You can customize this variable.