Nah.... not fanfic. Imperfect analysis. First thing I thought was stretchy pants with stirrups, but that stopped being fashionable in like 1989.

I couldn't tell if the other "strap" is a strap or part of the background, and considering her leg is straight, the stirrup should be under tension and flat, not bulged. Now if she's wearing the pants halfway down her ass, all bets are off.