/usr/share/doc/libgcab-dev/html is in libgcab-doc 0.7-2+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgcab-dev/../../gtk-doc/html/gcab
/usr/share/doc/libgcab-dev/../../gtk-doc/html/gcab