I thought of CSS - but that's still just using formatting, and will not work for everyone. Hmm.. maybe there could be a user setting that turns SPOILER into HTML comments as diotalevi had suggested I do - so that the majority surfing with a regular graphical browser can conveniently reveal the text by selecting it, and those using more "exotical" means have it concealed before the view the source. That would probably cover all bases.

Makeshifts last the longest.