Is there an Emacs Lisp library for generating HTML?

This is not quite what you're looking for, but there's a 20 minute video where a guy creates a simple website using UCW, the UnCommon Web application framework. It's all done in Emacs using lisp...

Here is a link to the transcript (all the code (~25 lines) is available at the end of the transcript).

This could be a starting point: http://www.emacswiki.org/emacs/HtmlLite

As you found out, xmlgen generates XML from a list structure. What I did find disappointing with the ``xmlgen` package that the format it supports is not quite the inverse of Emacs' xml parser.

I did add this to my copy of xmlgen:

;; this creates a routine to be the inverse of what xml-parse does
;;;###autoload
(defun xml-gen (form &optional in-elm level)
  "Convert a sexp to xml:
  '(p :class \"big\")) => \"<p class=\\\"big\\\" />\""
  (let ((level (or level 0)))
    (cond
     ((numberp form) (number-to-string form))
     ((stringp form) form)
     ((listp form)
      (destructuring-bind (xml attrs) (xml-gen-extract-plist form)
        (let ((el (car xml)))
          (unless (symbolp el)
            (error "Element must be a symbol (got '%S')." el))
          (setq el (symbol-name el))
          (concat "<" el (xml-gen-attr-to-string attrs)
                  (if (> (length xml) 1)
                      (concat ">" (mapconcat
                                   (lambda (s) (xml-gen s el (1+ level)))
                                   (cdr xml)
                                   "")
                              "</" el ">")
                    "/>"))))))))

(defun xml-gen-attr-to-string (plist)
  (reduce 'concat (mapcar (lambda (p) (concat " " (symbol-name (car p)) "=\"" (cdr p) "\"")) plist)))

(defun xml-gen-extract-plist (list)
  (list (cons (car list) (let ((kids (xml-node-children list)))
                           (if (= 1 (length kids))
                               kids
                             (remove-if-not 'listp kids))))
        (xml-node-attributes list)))

Note: the interface for this is xml-gen (not xmlgen which is the original parsing).

With this interface, the following holds:

(string-equal (xml-gen (car (xml-parse-region <some-region-of-xml>)))
              <some-region-of-xml>)

and

(equal (car (xml-parse-region (insert (xml-gen <some-xml-form>))))
       <some-xml-form>)

The new xml-gen does not strive to preserve the whitespace around that the xml-parse-region routine generates.