The document is out of date: PlantUML : Language Reference Guide, December 11, 2010 (Version 5737)
It is the doc I download from this website.
Your code has changed.
It do not understand: the end tag :</size> ; </color> I jusr remove and it work; and the tag <strike>, it do not understand!.
participant "The <b>Famous </b> Bob" as Bob
Alice -> Bob : A <i>well formated </i> message
note right of Alice
This is <size:18>displayed
<u>left of </u> Alice.
note left of Bob
This is <color :#118888 > displayed
<b><color:purple >left of Alice Bob </b>.
note over Alice , Bob
This is hosted by <img:sourceforge.jpg >
Have a nice day!