org-html-doctype

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

"xhtml-strict"

Documentation:
Document type definition to use for exported HTML files.
Can be set with the in-buffer HTML_DOCTYPE property or for
publishing, with :html-doctype.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.