(require-extension html-tags)

(define (html-transform list)
  (string-append
   (if (list? (car list))
       (<ul> (html-transform (car list)))
       (<li> (car list)))
   (if (not (null? (cdr list)))
       (html-transform (cdr list))
       "")))

(set! testlist '(one two (a b) three))
(pp (html-transform testlist))


Returns:

"<li>one</li><li>two</li><ul><li>a</li><li>b</li></ul><li>three</li>"