I'd rather have more official CSS classes. People can define their display any way they choose, with a default offered by the site (which may be very subtle). A special class for quotes from the previous posts, for example, or quotes from external docs.

You could have a special class for spoilers, where foreground and background are the same color. OTOH, behaviour on non-CSS compliant browsers is iffy. Perhaps you do need a <spoiler></spoiler> tag after all.