org stuff

collapsingtoc.css

#toggle-table-of-contents {
  display: none;
}

#toggle-table-of-contents ~ label {
  text-decoration: underline;
}

#toggle-table-of-contents ~ label::before {
  content: "hide ";
}

#toggle-table-of-contents:checked ~ label::before {
  content: "show ";
}

#text-table-of-contents {
  overflow-y: hidden;
}

#toggle-table-of-contents:checked ~ #text-table-of-contents {
  height: 0; /* could probably become a fancy transition */
  display: none;
}

org-html-toc.el

(defun org-html-toc (depth info)
  "Build a table of contents.
DEPTH is an integer specifying the depth of the table.  INFO is a
plist used as a communication channel.  Return the table of
contents as a string, or nil if it is empty."
  (let ((toc-entries
	 (mapcar (lambda (headline)
		   (cons (org-html--format-toc-headline headline info)
			 (org-export-get-relative-level headline info)))
		 (org-export-collect-headlines info depth)))
	(outer-tag (if (and (org-html-html5-p info)
			    (plist-get info :html-html5-fancy))
		       "nav"
		     "div")))
    (when toc-entries
      (concat (format "<%s id=\"table-of-contents\">\n" outer-tag)
	      (format "<h%d>%s</h%d>\n"
		      org-html-toplevel-hlevel
		      (org-html--translate "Table of Contents" info)
		      org-html-toplevel-hlevel)
          ;;
	      "<input id=\"toggle-table-of-contents\" type=\"checkbox\">"
	      "<label for=\"toggle-table-of-contents\">table of contents</label>"
          ;;
	      "<div id=\"text-table-of-contents\">"
	      (org-html--toc-text toc-entries)
	      "</div>\n"
	      (format "</%s>\n" outer-tag)))))