There isn't always enough entropy to uniquely identify a user - you can't rely on that to provide accurate tracking. If people are coming from a corporate environment using terminal servers then every single browser will identify as one user by that method.
Most browsers are now taking steps to stop the CSS custom styles hack working - for example this doesn't work in Chrome anymore. The website can show you which websites you've visited with the change in colour for the link state however it can no lonqer query it.