docbook-xml binary package in Ubuntu Focal i386

 DocBook is an XML document type definition (DTD). That is, it
 contains the "DocBook" document structure. This is used by authors
 or editors writing documents in the DocBook XML format. DocBook was
 designed for books, articles, or reference documentation about
 technical matters or software. It has emerged as an open, standard
 DTD in the software industry, and is the documentation system of
 choice for many free software projects.
 .
 This package contains the XML DTD for DocBook, which describes the
 formal structure for documents following this format. If you wish to
 author DocBook documents using SGML rather than XML, use the
 'docbook' package. You will need other packages in order to edit
 (psgml), validate (opensp, libxml2-utils) or format (docbook-xsl,
 docbook-dsssl) DocBook documents.
 .
 This package ships with the latest DocBook 4.5 XML DTD, as well as a
 selected set of legacy DTDs for use with older documents, including
 4.0, 4.1.2, 4.2, 4.3 and 4.4.

Source package