The summary property SHOULD be used as a fallback text representation, possibly automatically generated by the publisher. If there is no name property, the summary property SHOULD NOT include markup, and SHOULD be short enough to be used as a reasonable text representation of the object.

nameがないならマークアップつけないほうがいいっていうのはMastodonやAPどころか AS core で書かれてはいる

Activity Streams 2.0
https://www.w3.org/TR/activitystreams-core/#h-text-representations