# File lib/mail/message.rb, line 1556
    def html_part(&block)
      if block_given?
        @html_part = Mail::Part.new(&block)
        add_multipart_alternate_header unless html_part.blank?
        add_part(@html_part)
      else
        @html_part || find_first_mime_type('text/html')
      end
    end