The definition for the integrity attribute (Section 3.1) says:

"This metadata is generally encoded as a “named information” (ni) URI, as defined in RFC6920. [RFC6920]"

This is like saying it "... [may be] encoded as ...". Please change to something like:

"This metadata is MUST be encoded as a “named information” (ni) URI, as defined in RFC6920. [RFC6920]"

or

"This metadata is MUST be encoded as a URI, where, at a minimum, a user agent MUST support integrity metadata encoded as a named information” (ni) URI, as defined in RFC6920. [RFC6920]"