I'm not 100% sure how other mediawikis handle this, but Wikia dis-ables all inline styles, as certain things could break the mobile skin / be unreadable (a "floated" container, some set at 1000px width, etc). Only other html attributes are left, like class or id.
By default Wikia doesn't even support custom CSS styling in mobile even for classes. However, there are some ways to "hack" in mobile support (such as is done with the front page and css classes). One of the hacks is to load MediaWiki:Handheld.css on mobile (while it's not loaded on Wikia wikis without a "hack", it's the official page loaded on normal wikis, so most wikis use that page encase they enable it at a later date without hacks). A list of colors is at the bottom (those colors are intended for the colored boxes on High Sea Sage / Beastie Bay, but most likely won't change.
I suspect Wikia doesn't officially offer mobile CSS styling in handheld.css because they want to promote their mobile apps, and they don't support CSS (I'd strongly assume).
Edit:So the span shows up, just without and inline styles (this is "important" as things like div will still act like "block"s and not "inline-block"s;).
Since we know it means negative now, we should also include a "-" sign (as a fallback for wikia apps). We can also have it only show up on Wikia apps if you'd prefer using the classes "mobile-hide" and "desktop-hide" on the "-".