Rendering Greek/special characters

Hello,

I have to render the Greek character "Theta".
Using character entity Θ I get the appropriate symbol in HTML 
output (rendered by Mozilla).

- Can I rely on every new browser to render this character?

- PDF output, rendered by XEP, misses this character. What can I do?

- Where can I find a description/overview of what is contained in the 
"ISO-entity-sets" and which of these entitities I can use without 
worrying whether they are rendered or not?

Greetings,
Joachim

-- 
Joachim Ziegler                       Stuhlsatzenhausweg 85
Max-Planck-Institut für Informatik    66123 Saarbrücken
Email:           Germany