Install sgml-data by entering the following commands in the terminal:
sudo apt update sudo apt install sgml-data
Description:
common SGML and XML data
This package contains basic XML and SGML data shared and used by multiple packages. The package provides the technical format (Document Type Definition, or DTD) of all major versions of HTML. It also includes standard character entity definitions which describe spacial characters or international characters, as well as other standard miscellany used by other SGML and XML packages. . This package implements the Debian XML and SGML standards for entity placement and registration. This benefits the user by making the included materials instantly available to people writing, editing or building documentation without any further effort. See
Homepage: http://debian-xml-sgml.alioth.debian.org/
Version: 2.0.10
Section: universe/text