Yesterday, I came up with a way to generate the HTML details disclosure fairly easily using Org Special Blocks. I implemented that in ox-hugo. But I believe the same would be useful for ox-html too.
((string= block-type "details")
;; Recognize Org Special blocks like:
;; #+begin_details
;; summary. This will be wrapped in <summary> and </summary>
;; ---
;; details
;; #+end_details
(let* ((is-open (member "open" (org-element-property :attr_html special-block)))
(str1 (org-blackfriday-special-block special-block contents nil))
;; Insert a new-line before the closing </details> tag
;; for correct Markdown parsing for cases when the
;; Special Block content ends a code block. Without this
;; inserted newline, the Markdown converted content will
;; look like below, and Blackfriday won't parse it
;; correctly.
;; ```emacs-lisp
;; (message "a code block")
;; ```</details>
;; A closing </p> tag is also added.. the opening <p>
;; tag is later added in the `str2' var if summary is
;; present, else in `str3' var.
(str1 (replace-regexp-in-string "</details>\\'" "\n</p>\\&" str1))
;; Detect the summary divider special string "---". It
;; must begin at the beginning of a line. Also ensure to
;; replace only the first match, if any.
;; Also add the opening <p> tag with "details" class
;; so that just as CSS rules can be set for summary
;; ("details summary"), they can be set for the details
;; portion following the <summary> too, using "details
;; .details".
(str2 (replace-regexp-in-string
"^\\(---\\)\n\\(.\\|\n\\)*\\'"
"</summary><p class=\"details\">"
str1 nil nil 1))
(has-summary (not (string= str1 str2)))
str3)
;; (message "[DBG details/summary]: is-open:%S `%s' `%s'" is-open str1 str2)
(setq str3 (if has-summary
(replace-regexp-in-string "\\`<details>" "\\&<summary>" str2)
(replace-regexp-in-string "\\`<details>" "\\&<p class=\"details\">" str2)))
(if is-open
(replace-regexp-in-string "\\`\\(<details\\)>" "\\1 open>" str3)
str3)))
I used "---" as the summary/details divider so that even if someone exports that Org document to HTML without that support in ox-html, it exports as Summary — Details.
If this feature is approved for addition to ox-html, I can make a formal patch where instead of looking for "---", I look for the appropriate string based on the value of (plist-get info :with-special-strings).