Add an option to convert LaTeX fragments to HTML

* lisp/org.el (org-latex-to-html-convert-command): New option to
convert a LaTeX fragment directly into HTML.
(org-format-latex): Use the new option.
(org-format-latex-as-html): Do the conversion and return HTML.
* lisp/ox-html.el (org-html-with-latex): Document the 'html symbol.
(org-html-format-latex): This custom HTML conversion, like MathJax,
doesn't require preprocessing.
(org-html-latex-fragment): Use the new option.

This allows you to set a custom command
`org-latex-to-html-convert-command' that will take as input a LaTeX
fragment and use it to generate HTML for export.  This is very
open-ended in the sense that you can use any shell-command you
want.  This has been added in order to use latexml, but you could
use any other tool that generates HTML output text.

lisp/org.el

@@ -3198,6 +3198,22 @@ When using LaTeXML set this option to
 	  (const :tag "None" nil)
 	  (string :tag "\nShell command")))
 
+(defcustom org-latex-to-html-convert-command nil
+  "Command to convert LaTeX fragments to HTML.
+This command is very open-ended: the output of the command will
+directly replace the LaTeX fragment in the resulting HTML.
+Replace format-specifiers in the command as noted below and use
+`shell-command' to convert LaTeX to HTML.
+%i:     The LaTeX fragment to be converted.
+
+For example, this could be used with LaTeXML as
+\"latexmlc 'literal:%i' --profile=math --preload=siunitx.sty 2>/dev/null\"."
+  :group 'org-latex
+  :package-version '(Org . "9.5")
+  :type '(choice
+	  (const :tag "None" nil)
+	  (string :tag "\nShell command")))
+
 (defcustom org-preview-latex-default-process 'dvipng
   "The default process to convert LaTeX fragments to image files.
 All available processes and theirs documents can be found in
@@ -15617,6 +15633,10 @@ Some of the options can be changed using the variable
 		    (if (string= (match-string 0 value) "$$")
 			(insert "\\[" (substring value 2 -2) "\\]")
 		      (insert "\\(" (substring value 1 -1) "\\)"))))
+		 ((eq processing-type 'html)
+		  (goto-char beg)
+		  (delete-region beg end)
+		  (insert (org-format-latex-as-html value)))
 		 ((assq processing-type org-preview-latex-process-alist)
 		  ;; Process to an image.
 		  (cl-incf cnt)
@@ -15782,6 +15802,15 @@ inspection."
       ;; Failed conversion.  Return the LaTeX fragment verbatim
       latex-frag)))
 
+(defun org-format-latex-as-html (latex-frag)
+  "Convert LaTeX to HTML with a custom conversion command.
+`LATEX-FRAG' is the latex fragment
+Set the custom command with `org-latex-to-html-convert-command'."
+  (let ((cmd (format-spec org-latex-to-html-convert-command
+			  `((?i . ,latex-frag)))))
+    (message "Running %s" cmd)
+    (setq shell-command-output (shell-command-to-string cmd))))
+
 (defun org--get-display-dpi ()
   "Get the DPI of the display.
 The function assumes that the display has the same pixel width in

lisp/ox-html.el

@@ -775,6 +775,8 @@ e.g. \"tex:mathjax\".  Allowed values are:
   `verbatim'    Keep everything in verbatim
   `mathjax', t  Do MathJax preprocessing and arrange for MathJax.js to
                 be loaded.
+  `html'        Use `org-latex-to-html-convert-command' to convert
+                LaTeX fragments to HTML.
   SYMBOL        Any symbol defined in `org-preview-latex-process-alist',
                 e.g., `dvipng'."
   :group 'org-export-html
@@ -2769,12 +2771,13 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
 (defun org-html-format-latex (latex-frag processing-type info)
   "Format a LaTeX fragment LATEX-FRAG into HTML.
 PROCESSING-TYPE designates the tool used for conversion.  It can
-be `mathjax', `verbatim', nil, t or symbols in
+be `mathjax', `verbatim', `html', nil, t or symbols in
 `org-preview-latex-process-alist', e.g., `dvipng', `dvisvgm' or
 `imagemagick'.  See `org-html-with-latex' for more information.
 INFO is a plist containing export properties."
   (let ((cache-relpath "") (cache-dir ""))
-    (unless (eq processing-type 'mathjax)
+    (unless (or (eq processing-type 'mathjax)
+                (eq processing-type 'html))
       (let ((bfn (or (buffer-file-name)
 		     (make-temp-name
 		      (expand-file-name "latex" temporary-file-directory))))
@@ -2888,6 +2891,8 @@ CONTENTS is nil.  INFO is a plist holding contextual information."
     (cond
      ((memq processing-type '(t mathjax))
       (org-html-format-latex latex-frag 'mathjax info))
+     ((memq processing-type '(t html))
+      (org-html-format-latex latex-frag 'html info))
      ((assq processing-type org-preview-latex-process-alist)
       (let ((formula-link
 	     (org-html-format-latex latex-frag processing-type info)))