alt Hacker NewsI thought about that but to your point CSS is cachable. In fact I made mine immutable. No I just visually spot patterns and use that to study other facets of the agent, other client headers or lack thereof, supported protocol, accepted encoding and so on.
Maybe it is enough to include some css/js which is served without cache and is loaded after all user visible css/js is loaded. Make it small enough to not cause too much bandwidth for the server and legitimate clients. Then anyone who doesn’t hit that CSS file gets banned.