That unvalid feed will allow some non compliant OPDS client to perform search.
Note that Mantano Reader is the only reader so far that works with COPS's
perfectly valid feed.
FBReaderJ was working fine in May 2012 ans is not working anymore : see https://github.com/geometer/FBReaderJ/issues/70
Moon+ Reader is also not working without modification.
Not really happy about that. Fix#14