sites_dtd

      1| <!ELEMENT sites (browser1, browser2?, browser3?, site+)>
      2| <!ELEMENT browser1 (#PCDATA)>
      3| <!ELEMENT browser2 (#PCDATA)>
      4| <!ELEMENT browser3 (#PCDATA)>
      5| <!ELEMENT site (browser, url, description, password?)>
      6| <!ELEMENT browser (#PCDATA)>
      7| <!ELEMENT url (#PCDATA)>
      8| <!ELEMENT description (#PCDATA)>
      9| <!ELEMENT password (#PCDATA)>