(define button-a
  (sxml-zipper `(button (@ (class "button")) "foo")))

(define button-b
  (sxml-zipper `(button (@ (class "red") (id "foo")) "hi")))

(define (find-class-attr x)
  (and-let* ((x (find-first x (tag= '@))))
    (find-first x (tag= 'class))))

(define (merge-classes a b)
  (and-let* ((x (text (find-class-attr a)))
             (y (find-class-attr b))
             (y (edit y (lambda (n) (cons* (car n) x " " (cdr n))))))
    (root y)))

;;; usage:

(node (merge-classes button-a button-b))
=>
(button (@ (class "button" " " "red") (id "foo")) "hi")