let $doc := <html><head><title>Example Title</title></head><body><p>Paragraph</p></body></html>return serialize($doc, map{"method":"xhtml", "version":"5.0"})
Use
serialize($doc, map{"method":"xhtml", "html-version":"5.0"})
Note however that XHTML usually means the HTML elements are in the XHTML namespace i.e.
let $doc := <html
xmlns="http:/www.w3.org/1999/xhtml"><head><title>Example
Title</title></head><body><p>Paragraph</p></body></html>
return serialize($doc, map{"method":"xhtml",
"html-version":"5.0"})