aemdliz, the <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
Xara writes your HTML for you so its declaration should be immutable.

We have:
XHTML 1.0 Transitional: This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
It is probably this to ensure maximal coverage for browsers in use.

Yes, you could use a text editor after publishing to alter it but it would revert every time you re-publish.

Acorn