ah, I didn't think of .hidden-xs -- but wait, I actually don't want to hide at '.xs' size, but only at SMALLER than .xs size. I'll look around if there's a pre-build class for that. 

Wrapping selectors in media query is standard CSS.  Wrapping media queries in selector is a SASS enhancement, which compiles to the former (and which I think BL already uses in a bunch of places, it's often convenient).   In neither case does `@extend .sr-only` work combined with a media query, i did try both just in case. http://designshack.net/articles/css/sass-and-media-queries-what-you-can-and-cant-do/