What is this sentence talking about?
Topic on Talk:Design/Typography
It means you do not need two forms of emphasis. Italic is one; bold is another; bold italic is two emphasisesessesss.
But where? This is surely false in Wikipedia articles, so if you attempted to proclaim it as the 11th commandment you failed.
I would assume that means "In the interface and documentation" only - because the article content is a different kettle of fish - but you have a good (implied but unstated) point about those 2 areas being often-distinct, and this project's description currently not clarifying that.
I think that "Shift & emphasis" is the only section on the page that requires amendment, so I'll boldly add that now.
It means that one shift is sufficient for emphasis, we don't need to bold & italicize text. Two shifts create noise which hinders from the reading experience. Vibhabamba (talk) 22:45, 4 February 2014 (UTC)
Just to clarify, the confusion was because: within English Wikipedia articles (and possibly/probably elsewhere), we do need and intend to use bold&italic at the same time, eg in en:Macbeth the the first word is italic because it's a title of an artwork, but is also bold because it's the article topic. (all per en:MOS:TEXT guidelines)
I think we all agree that this is fine, and the clarifications in this project-page were purely for UI and documentation elements, as per my edit. HTH.