Template talk:Spoiler

The mouse events aren't working because they appear here as if they are HTML attributes; they are not. Mouse events like onMouseOver and onMouseOut are Javascript, not HTML. You would have to use external javascript to make that work, as in MediaWiki:Common.js.

However, the easier way these days would be with CSS. Just add a class like  to the template:

...and a style definition to Special:CSS like: I'm confident that would work... but I make typos and I haven't tested it :D  —［ iynque  ］［  Talk |  Contributions  | 20:06, 5 Mar 2016 ］