Install xhtml-relaxng by entering the following commands in the terminal:
sudo apt update sudo apt install xhtml-relaxng
Description:
Implementation in RELAX NG of the modularization of XHTML
This package contains an implementataion in RELAX NG of the modularization of XHTML specified by the Modularization of XHTML Recommendation . The RELAX NG schema has not been mechanically derived from the DTDs. Rather, it has been hand-crafted so as to produce a good quality RELAX NG schema. However, it may well contain bugs.
Homepage: https://github.com/relaxng/jing-trang
Version: 20151127+dfsg-1
Section: universe/misc