Middleware that prefixes css on-the-fly. Intended to be used in a development setting with the express.static middleware, but should work with any middleware further down the stack, even an http proxy. autoprefixer plays nice with conditional GET. If the original response has an ETag, autoprefixer will add to it so the ETag of the compiled response never clashes with the original ETag. That prevents the middleware issuing the original response from being confused into sending a false positive 304 Not Modified if autoprefixer is turned off or removed from the stack later.