Comment 3 for bug 1339766

what validator are you refering to? The Check Book function in the
editor, that is not a validator. That checks your book for things
that are likely to not work on most reader devices. And using remote video
resources most definitely is likely to not work.

Also, having href on manifest items (I ssume you meam <item> tags) is
perfectly valid. The problem is having hrefs that refer to resources not
inside the book, which is what the error message states "A file listed
in the manifest is missing, you should either remove it from the
manifest or add the missing file to the book"

 status invalid