!important
Fragment of a discussion from User talk:Rua
You could also use the universal selector to explicitly select an element and all of its descendants. Is is simple and will override unanticipated italics in elements other than i and em, but still can be overriden by still-more specific declarations and in inline CSS. It could still break in nested languages.
.Cyrl, .Cyrl *{ font-style: normal; }