Comment pseudo-elements
           These are element objects with a "$h->tag" value of "~comment", and
           the content of the comment is stored in the "text" attribute
           ("$h->attr ("text")").  For example, parsing this code with
           HTML::TreeBuilder...

             <!-- I like Pie.
                Pie is good
             -->

           produces an HTML::Element object with these attributes:

             "_tag",
             "~comment",
             "text",
             " I like Pie.\n     Pie is good\n  "