Hello,
I can create <span style='whatever'> buttons, but NOT <spoiler>. To be precise, when I create it, by dutifully filling caption spoiler, before <spoiler> and after </spoiler>, as soon as I save, it only keeps the Caption and empties the two others. The button appears with the right name but does nothing.
http://wordpress.org/extend/plugins/post-editor-buttons-fork/