Some examples of using Plump.

plump-use.lisp

(defun parse-tree/fix-anchors (tree base-url)
  "Fix anchor URLs in the `TREE' to work correctly with the <base> tag."
  (plump:traverse tree
                  (lambda (node)
                    (when (starts-with-subseq "#" (plump:attribute node "href"))
                      (setf (plump:attribute node "href") (concatenate 'string base-url (plump:attribute node "href")))))
                  :test (lambda (node)
                          (and (plump:element-p node)
                               (string-equal (plump:tag-name node) "a")))))

(defun parse-tree/extract-urls (tree)
  (let ((urls))
    (plump:traverse tree
                    (lambda (node)
                      (let ((url (or (plump:attribute node "href")
                                     (plump:attribute node "src"))))
                        (when url
                          (pushnew url urls :test #'string=))))
                    :test (lambda (node)
                            (and (plump:element-p node)
                                 (member (plump:tag-name node)
                                         '("a" "img")
                                         :test #'string-equal))))
    urls))