Font formatting with Creole (font size colour etc.): see: http://plantuml.com/creole
@startuml
skinparam handwritten true
skinparam RectangleBackgroundColor white
rectangle "This is **bold**"
rectangle "This is //italics//"
rectangle "This is ""monospaced"" "
rectangle "This is --stricken-out--"
rectangle " This is __underlined__"
rectangle " This is ~~wave-underlined~~"
@enduml