html4, xhtml or html5 (default). For
compatibility reasons, html is accepted as an
alias for html4.<|DOCTYPE DocType > line for page//1 and
page//2.Content-type for reply_html_page/3
Note that the doctype and content_type flags are covered by
distinct prolog flags: html4_doctype, xhtml_doctype and
html5_doctype and similar for the content type. The Dialect
must be switched before doctype and content type.