Existing Wording: Since that are not contained in the resource cannot be guaranteed to be be available when the resource is presented to a user, the source for any images that are an essential part of the narrative SHOULD always be embedded as a data: url, in an attachment or a contained resource.

Proposed Wording: Since IDs that are not contained in the resource cannot be guaranteed to be be available when the resource is presented to a user, the source for any images that are an essential part of the narrative SHOULD always be embedded as a data: url, in an attachment or a contained resource.

Comment:

Missing "IDs"?