[BUG] ox-html output does not pass validation for html4-strict doctype [9.7-pre (release_9.6.23-1423-gcea6a1.dirty @ /home/yantar92/.emacs.d/straight/build/org/)]

[Ihor Radchenko]

Setting (setq org-html-doctype "html4-strict") and exporting a trivial
Org document (like the attached) to HTML fails to pass validation at
https://validator.w3.org/

The error is

 Line 6, Column 72: character data is not allowed here
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8" />

 ....using XHTML-style self-closing tags (such as <meta ... />) in HTML
 4.01 or earlier. To fix, remove the extra slash ('/') character. For
 more information about the reasons for this, see Empty elements in
 SGML, HTML, XML, and XHTML.

caused by /> self-closing tag.

However, dropping /> will lead to validation failing for the default
value of `org-html-doctype' - "xhtml-strict".

I am wondering whether it is at all possible to use the same syntax and
yet pass validation for all the allowed values of `org-html-doctype':
"html4-strict", "html4-transitional", "html4-frameset", "xhtml-strict",
"xhtml-transitional", "xhtml-frameset", "xhtml-11", "html5", "xhtml5".

20240322205907-testmath.org
Description: Lotus Organizer

Emacs  : GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.41, 
cairo version 1.18.0)
 of 2024-03-24
Package: Org mode version 9.7-pre (release_9.6.23-1423-gcea6a1.dirty @ 
/home/yantar92/.emacs.d/straight/build/org/)
-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>