/usr/share/doc/kdiff3-doc/html/common is in kdiff3-doc 0.9.97-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/kdiff3-doc/html/../../kde/HTML/en/common
/usr/share/doc/kdiff3-doc/html/../../kde/HTML/en/common