Depends on how you interpret the osmc:symbol definition. It was never really defined what an empty background exactly is supposed to mean. Background is not optional. I’d argue that the way US trails use it, it reassigns the <foreground> placeholder the meaning of background. That seems a little bit odd and a workaround for the fact that no *_stripe background is defined. It gets even odder, when as a result foreground2 is needed to describe additional colors.
I’d rather go for a simple white:white_stripe.