ox-html: Fix CUSTOM_ID handling

* lisp/ox-html.el (org-html-headline): Prefer raw CUSTOM_ID over
  internal reference.

This is required for custom CSS and to link generated page from
outside Org.  However, the user is responsible for not using forbidden
characters in its value.

lisp/ox-html.el

@@ -2444,18 +2444,26 @@ holding contextual information."
            (full-text (funcall (plist-get info :html-format-headline-function)
                                todo todo-type priority text tags info))
            (contents (or contents ""))
-           (reference (org-export-get-reference headline info))
-           (extra-id (let ((id (org-element-property :ID headline)))
-		       (if (not id) ""
-			 (org-html--anchor (concat "ID-" id) nil nil info)))))
+	   (ids (delq nil
+                      (list (org-element-property :CUSTOM_ID headline)
+                            (org-export-get-reference headline info)
+                            (org-element-property :ID headline))))
+           (preferred-id (car ids))
+           (extra-ids
+	    (mapconcat
+	     (lambda (id)
+	       (org-html--anchor
+		(if (org-uuidgen-p id) (concat "ID-" id) id)
+		nil nil info))
+	     (cdr ids) "")))
       (if (org-export-low-level-p headline info)
           ;; This is a deep sub-tree: export it as a list item.
           (let* ((type (if numberedp 'ordered 'unordered))
                  (itemized-body
                   (org-html-format-list-item
                    contents type nil info nil
-                   (concat (org-html--anchor reference nil nil info)
-                           extra-id
+                   (concat (org-html--anchor preferred-id nil nil info)
+                           extra-ids
                            full-text))))
             (concat (and (org-export-first-sibling-p headline info)
                          (org-html-begin-plain-list type))
@@ -2474,8 +2482,8 @@ holding contextual information."
                           extra-class)
                   (format "\n<h%d id=\"%s\">%s%s</h%d>\n"
                           level
-                          reference
-                          extra-id
+                          preferred-id
+                          extra-ids
                           (concat
                            (and numberedp
                                 (format