Quoting Steve Summit <scs(a)eskimo.com>om>:
I know this has been proposed before, but: the obvious
solution
is to have spoiler warnings enabled by default, but with a
"dismiss" box, just like WMF fund drive announcement banners
and the like. No mucking about with user css files required.
I like this also, but I think this would require changing the MediaWiki
settings.