[1] Activity Streams 2.0 (2017-05-21 23:50:36 +09:00) <https://w3c.github.io/activitystreams/core/#dfn-link>
[2] Activity Vocabulary (2017-05-21 23:50:36 +09:00) <https://w3c.github.io/activitystreams/vocabulary/#dfn-link>