anchor.diff:  (defun valid-attribute? (tag name)
     (or (null tag)                        ;A dynamic tag.
         (unvalidated-attribute? name)
  +      ;; Don't try to validate attributes on custom elements.
  +      (valid-custom-element-name? tag)
         (eql name :attrs)
         (global-attribute? name)
         (aria-attribute? name)
  
positive.code: 
  (defun aria-attribute? (name)
    (memq name *aria-attributes*))
  
negative.code:     :track :u :ul :var :video :wbr))
  
  (defun valid? (element)
    (or (memq element *html5-elements*)
        (valid-custom-element-name? element)))
  
  (defun invalid? (element)
    (not (valid? element)))
  