ox-html: Add customizable format string for equations

* lisp/ox-html.el (org-html-equation-reference-format): New variable.
* doc/org-manual.org update to reference new variable

doc/org-manual.org

@@ -15851,6 +15851,7 @@ Settings]]), however, override everything.
 | ~:html-link-use-abs-url~                       | ~org-html-link-use-abs-url~                       |
 | ~:html-mathjax-options~                        | ~org-html-mathjax-options~                        |
 | ~:html-mathjax-template~                       | ~org-html-mathjax-template~                       |
+| ~:html-equation-reference-format~              | ~org-html-equation-reference-format~              |
 | ~:html-metadata-timestamp-format~              | ~org-html-metadata-timestamp-format~              |
 | ~:html-postamble-format~                       | ~org-html-postamble-format~                       |
 | ~:html-postamble~                              | ~org-html-postamble~                              |

etc/ORG-NEWS

@@ -384,6 +384,15 @@ now. E.g.,
 This bug [[https://lists.gnu.org/archive/html/emacs-orgmode/2013-08/msg00072.html][originally reported]] by Matt Lundin and investigated by Andrew
 Hyatt has been fixed.  Thanks to both of them.
 
+*** Format of equation reference in HTML export can be specified
+
+By default, HTML (via MathJax) and LaTeX export equation references
+using different commands. LaTeX must use ~\ref{%s}~ because it is used
+for all labels; however, HTML (via MathJax) uses ~\eqref{%s}~ for
+equations producing inconsistent output. New option
+~org-html-equation-reference-format~ sets the command used in HTML
+export.
+
 * Version 9.3
 
 ** Incompatible changes

lisp/ox-html.el

@@ -121,6 +121,7 @@
     (:html-link-home "HTML_LINK_HOME" nil org-html-link-home)
     (:html-link-up "HTML_LINK_UP" nil org-html-link-up)
     (:html-mathjax "HTML_MATHJAX" nil "" space)
+    (:html-equation-reference-format "HTML_EQUATION_REFERENCE_FORMAT" nil org-html-equation-reference-format t)
     (:html-postamble nil "html-postamble" org-html-postamble)
     (:html-preamble nil "html-preamble" org-html-preamble)
     (:html-head "HTML_HEAD" nil org-html-head newline)
@@ -761,6 +762,24 @@ The function should return the string to be exported."
 
 ;;;; LaTeX
 
+(defcustom org-html-equation-reference-format "\\eqref{%s}"
+  "The MathJax command to use when referencing equations.
+
+This is a format control string that expects a single string argument
+specifying the label that is being referenced. The argument is
+generated automatically on export.
+
+The default is to wrap equations in parentheses (using \"\\eqref{%s}\)\".
+
+Most common values are:
+
+  \\eqref{%s}    Wrap the equation in parentheses
+  \\ref{%s}      Do not wrap the equation in parentheses"
+  :group 'org-export-html
+  :package-version '(Org . "9.4")
+  :type 'string
+  :safe t)
+
 (defcustom org-html-with-latex org-export-with-latex
   "Non-nil means process LaTeX math snippets.
 
@@ -3113,9 +3132,9 @@ INFO is a plist holding contextual information.  See
                     (eq 'latex-environment (org-element-type destination))
                     (eq 'math (org-latex--environment-type destination)))
                ;; Caption and labels are introduced within LaTeX
-	       ;; environment.  Use "eqref" macro to refer to those in
-	       ;; the document.
-               (format "\\eqref{%s}"
+	       ;; environment.  Use "ref" or "eqref" macro, depending on user
+               ;; preference to refer to those in the document.
+               (format (plist-get info :html-equation-reference-format)
                        (org-export-get-reference destination info))
              (let* ((ref (org-export-get-reference destination info))
                     (org-html-standalone-image-predicate