it's a browser problem. I've just tested with my current one (Mozilla 1.1) and everything worked as expected ("span.addlinks { font-size: 20px;}")
you should had told us at least tell what browser/version are you using =\
I bet it's Netscape 4.x - I've had problems galore with that sucker. And it's the "standard browser" at work. Lucky for me that Phoenix installs in one directory, I installed it without Win2K admin privleges. :) Happy Moz surfing on a AIX box, emulating Win2K, with Phoenix clandestinely installed.