okular-extra-backends seems to be only necessary when you use the the KHTMLPart widget which is not the default.
IMHO kchmviewer should recommend okular-extra-backends instead of depending on it. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org