He paid, so I did put up a link to his
http://minefield.bitcoinlab.org link. This site is safe, right? It's down now.
Accepting ads that are anything more than a picture, alt text and a URL doesn't seem all that safe; specially considering how tempting of a target users of the forum are...
They're safe when someone is manually reviewing them. It actually wouldn't be all that difficult to automatically verify that ads are OK: CSS can never be a security risk, and a small whitelist of known-safe HTML tags and attributes would prevent other attacks. I may add automatic verification if I ever automate the ad system, though some sort of manual approval will always be required because the ad content and size also need to be checked. (Automatically checking an ad's actual screen size seems difficult.)
HTML/CSS ads are much smaller byte-wise; they can be seen by text browsers, search engines, and the visually-impaired; people can deal with them more naturally (copy/paste, etc.); they can do things that images can't do; and ad blockers can't block them as easily. They are clearly superior to image ads in almost every way.