Translations:Manual:$wgSpamRegex/63/en

This has given spammers the opportunity to invent a sneaky trick to hide their spam from view. It doesn't show up on your pages, but it does show up in your edit boxes, and the changes show up in your 'recent changes' display. As such it causes confusion to your legitimate users, and that's before you consider the effects of helping a spammer by hosting their links. Generally 'CSS Hidden Spam' is all bad. Just because you can't see it (easily), doesn't mean you can ignore it.