diff --git a/fep/521a/fep-521a.md b/fep/521a/fep-521a.md
index 18537dd..f34bfbf 100644
--- a/fep/521a/fep-521a.md
+++ b/fep/521a/fep-521a.md
@@ -36,6 +36,8 @@ The `Multikey` object MAY contain the `expires` property indicating the expirati
 
 ### Key IDs
 
+Key identifiers MUST be absolute [URIs][RFC-3986].
+
 Identifiers SHOULD be generated by appending a fragment identifier to the actor ID. That allows consumers to retrieve both actor object and associated keys with a single HTTP request. If actor's keys are identified using fragment identifiers, each key MUST have a unique fragment identifier.
 
 Resolution of a URI containing a fragment identifier is performed using the algorithm specified in section *3.4 Fragment Resolution* of [Controlled Identifiers][FragmentResolution] specification.
@@ -46,7 +48,7 @@ Implementers can use cryptographic keys of any type for which a [Multicodec] pre
 
 ## Adding keys to an actor object
 
-`Multikey` objects MUST be added to the actor object, which is considered a controlled identifier document, as described in [Controlled Identifiers][ControlledIdentifiers] specification.
+`Multikey` objects are added to the actor object, which is considered a controlled identifier document, as described in [Controlled Identifiers][ControlledIdentifiers] specification.
 
 If the key is intended to be used for signing ActivityPub objects, it MUST be added to the [`assertionMethod`][Assertion] array in the actor object.
 
@@ -54,6 +56,8 @@ Other use cases are out of scope of this proposal.
 
 Implementations are discouraged from adding any objects to the `assertionMethod` array that do not conform to this specification. Implementations encountering non-conformant entries in the `assertionMethod` array SHOULD ignore them.
 
+An actor MAY have no associated public keys.
+
 ### Example
 
 ```json
@@ -95,16 +99,18 @@ See [fep-521a.feature](./fep-521a.feature)
 - S. Bradner, [Key words for use in RFCs to Indicate Requirement Levels][RFC-2119], 1997
 - Dave Longley, Manu Sporny, Markus Sabadello, Drummond Reed, Orie Steele, Christopher Allen, [Controlled Identifiers v1.0][ControlledIdentifiers], 2025
 - Protocol Labs, [Multicodec][Multicodec]
+- T. Berners-Lee, R. Fielding, L. Masinter, [Uniform Resource Identifier (URI): Generic Syntax][RFC-3986], 2005
 
 [ActivityPub]: https://www.w3.org/TR/activitypub/
 [SecurityVocabulary]: https://www.w3.org/2025/credentials/vcdi/vocab/v2/vocabulary.html
-[RFC-2119]: https://tools.ietf.org/html/rfc2119.html
+[RFC-2119]: https://datatracker.ietf.org/doc/html/rfc2119.html
 [ControlledIdentifiers]: https://www.w3.org/TR/cid-1.0/
 [Multikey]: https://www.w3.org/TR/cid-1.0/#Multikey
 [Multibase]: https://www.w3.org/TR/cid-1.0/#multibase-0
 [Assertion]: https://www.w3.org/TR/cid-1.0/#assertion
 [FragmentResolution]: https://www.w3.org/TR/cid-1.0/#fragment-resolution
 [Multicodec]: https://github.com/multiformats/multicodec/
+[RFC-3986]: https://datatracker.ietf.org/doc/html/rfc3986
 
 ## Copyright