ox-html: Make it possible to self link headlines

* lisp/ox-html.el (org-html-self-link-headlines): New variable.
(org-html-headline): Create a hyperlink on headlines
when :html-self-link-headlines is set.

doc/org-manual.org

@@ -12531,6 +12531,21 @@ For larger raw HTML code blocks, use these HTML export code blocks:
 ,#+END_EXPORT
 #+end_example
 
+*** Headlines in HTML export
+:PROPERTIES:
+:DESCRIPTION: Formatting headlines.
+:END:
+#+cindex: headlines, in HTML export
+
+Headlines are exported to =<h1>=, =<h2>=, etc.  Each headline gets the
+=id= attribute from =CUSTOM_ID= property, or a unique generated value,
+see [[*Internal Links]].
+
+#+vindex: org-html-self-link-headlines
+When ~org-html-self-link-headlines~ is set to a non-~nil~ value, the
+text of the headlines is also wrapped in =<a>= tags.  These tags have
+a =href= attribute making the headlines link to themselves.
+
 *** Links in HTML export
 :PROPERTIES:
 :DESCRIPTION: Inserting and formatting links.
@@ -15660,6 +15675,7 @@ Settings]]), however, override everything.
 | ~:html-postamble~                              | ~org-html-postamble~                              |
 | ~:html-preamble-format~                        | ~org-html-preamble-format~                        |
 | ~:html-preamble~                               | ~org-html-preamble~                               |
+| ~:html-self-link-headlines~                    | ~org-html-self-link-headlines~                    |
 | ~:html-table-align-individual-field~           | ~de{org-html-table-align-individual-fields~       |
 | ~:html-table-attributes~                       | ~org-html-table-default-attributes~               |
 | ~:html-table-caption-above~                    | ~org-html-table-caption-above~                    |

etc/ORG-NEWS

@@ -403,6 +403,11 @@ beginning of a headline when using Org speed commands.  Now, if there
 is already a restriction at point, hitting =<= again (or =C-x C-x <=) will
 remove it.
 
+*** Headlines can now link to themselves in HTML export
+
+When enabling ~org-html-self-link-headlines~ the headlines exported to
+HTML contain a hyperlink to themselves.
+
 ** New commands and functions
 
 *** ~org-insert-structure-template~

lisp/ox-html.el

@@ -152,6 +152,7 @@
     (:html-metadata-timestamp-format nil nil org-html-metadata-timestamp-format)
     (:html-postamble-format nil nil org-html-postamble-format)
     (:html-preamble-format nil nil org-html-preamble-format)
+    (:html-self-link-headlines nil nil org-html-self-link-headlines)
     (:html-table-align-individual-fields
      nil nil org-html-table-align-individual-fields)
     (:html-table-caption-above nil nil org-html-table-caption-above)
@@ -802,6 +803,13 @@ but without \"name\" attribute."
   :package-version '(Org . "8.0")
   :type 'boolean)
 
+(defcustom org-html-self-link-headlines nil
+  "When non-nil, the headlines contain a hyperlink to themselves."
+  :group 'org-export-html
+  :package-version '(Org . "9.3")
+  :type 'boolean
+  :safe #'booleanp)
+
 ;;;; Inlinetasks
 
 (defcustom org-html-format-inlinetask-function
@@ -2592,7 +2600,11 @@ holding contextual information."
                                todo todo-type priority text tags info))
            (contents (or contents ""))
 	   (id (or (org-element-property :CUSTOM_ID headline)
-		   (org-export-get-reference headline info))))
+		   (org-export-get-reference headline info)))
+	   (formatted-text
+	    (if (plist-get info :html-self-link-headlines)
+		(format "<a href=\"#%s\">%s</a>" id full-text)
+	      full-text)))
       (if (org-export-low-level-p headline info)
           ;; This is a deep sub-tree: export it as a list item.
           (let* ((html-type (if numberedp "ol" "ul")))
@@ -2603,7 +2615,7 @@ holding contextual information."
 	     (org-html-format-list-item
 	      contents (if numberedp 'ordered 'unordered)
 	      nil info nil
-	      (concat (org-html--anchor id nil nil info) full-text)) "\n"
+	      (concat (org-html--anchor id nil nil info) formatted-text)) "\n"
 	     (and (org-export-last-sibling-p headline info)
 		  (format "</%s>\n" html-type))))
 	;; Standard headline.  Export it as a section.
@@ -2630,7 +2642,7 @@ holding contextual information."
                                  "<span class=\"section-number-%d\">%s</span> "
                                  level
                                  (mapconcat #'number-to-string numbers ".")))
-                           full-text)
+                           formatted-text)
                           level)
                   ;; When there is no section, pretend there is an
                   ;; empty one to get the correct <div