Marking up mathematical theorems

Hello,

Short question:

What's a good way of marking up mathematical theorems?
(Marked up theorems should be processed specially by the 
XSLT-stylesheets, e.g. by printing them in italics.)

Greetings,
Joachim

-- 
Joachim Ziegler                       Stuhlsatzenhausweg 85
Max-Planck-Institut für Informatik    66123 Saarbrücken, Germany
Email:           Tel.: (+49) 0681 9325-127