DOCBOOK: problems with Davenport//DTD DocBook V3.0

hi!
I've got some problems with debian's docbook packages

nsgmls -s C-editing-with-VIM-HOWTO.sgml
nsgmls:C-editing-with-VIM-HOWTO.sgml:140:17:E: element "MEDIAOBJECT" undefined
nsgmls:C-editing-with-VIM-HOWTO.sgml:142:20:E: element "IMAGEOBJECT" undefined
nsgmls:C-editing-with-VIM-HOWTO.sgml:144:23:E: there is no attribute "FILEREF"
nsgmls:C-editing-with-VIM-HOWTO.sgml:144:44:E: there is no attribute "FORMAT"
nsgmls:C-editing-with-VIM-HOWTO.sgml:144:49:E: element "IMAGEDATA" undefined
nsgmls:C-editing-with-VIM-HOWTO.sgml:151:16:E: element "TEXTOBJECT" undefined
nsgmls:C-editing-with-VIM-HOWTO.sgml:161:10:E: end tag for "FIGURE" which is not finished

the identifier is:
<!DOCTYPE Article PUBLIC "-//Davenport//DTD DocBook V3.0//EN">

but I've noticed this:

%Changes from DocBook V3.0 to DocBook V3.1:
[...]
%- Added QAndASet, MediaObject, and InlineMediaObject.
                   ^^^^^^^^^^^
what's the matter?

-- 
best regards
riccardo fabris