“The only way that is known to me currently is, to disable CSS for a webpage completely (you can do this with a plugin like uMatrix). The problem that almost every modern webpage looks very ugly without CSS and is sometimes even unusable completely. So, disable CSS is not a real option, except when you are very worried about your privacy (for example, when you are using Tor browser, you should maybe disable CSS).
A better solution would be, that browsers does not load the external content (referenced in CSS), when it is needed, but when the site is loaded. Then it would be impossible to detect single actions. This modification to content loading could be implemented by the browsers itself, or maybe by a plugin (similar to NoScript or uMatrix)
The problem is that this solution maybe has a performance impact, because the browser has to load a lot of content on initial site loading (and maybe the browser will not use the content at all).”