I'm unfortunately having trouble finding a Wiki that fits all these criteria: a) not too complicated (one file is best), b) has reasonable default CSS classes and structure, and c) allows HTML tags without allowing dangerous tags.

UseModWiki, and its cousin OddMuseWiki came close, but failed on point b). Twiki would be great, but fails on point a). I think Kwiki might be the one that wins, but it's more of a framework than a one-script Wiki, and I therefore haven't installed it. Eventually, I'll give it a shot to see.

I fear that in order to have the features I need, I may end up writing my own Wiki in Ruby. I'm sure that would result in my maintaining two feature-incomplete varieties of Web-management software: my gemcast weblog engine, and a new Ruby-based Wiki engine.