If <<Composite>> is used with rectangle, it bolds all the text for some reason? On purpose? Not very critical but strange...
@startuml
title rectangle text is bolded if Composite used
rectangle "Composite" as Composite <<Composite>>
rectangle "Observer" as Observer <<Observer>>
rectangle "Subject" as Subject <<Subject>>
@enduml
@startuml
title this works
hide circle
hide methods
hide attributes
class "Composite" as Composite <<Composite>>
class "Observer" as Observer <<Observer>>
class "Subject" as Subject <<Subject>>
@enduml