[css-env-1] Avoid fingerprinting using environment variables

[SebastianZ]

With exposing different environment variables comes the risk of being able to identify the user agent, operating system, or device being used.

E.g. by analyzing the different values of the safe-area-inset-* variables, someone can draw conclusions to which device is used as some devices have specific values for them.

Similarily, if the device's body color (#5826) or a user agent's accent color (#5900) are exposed as environment variables, those could be used to identify the device or user agent used.

I'm not sure whether there is actually a way to completely avoid fingerprinting, though it should at least be mitigated in some way. One way would be to not expose their values directly, e.g. by computing the env() function to itself.

In the end, there also needs to be a privacy and security section outlining the possible fingerprinting risks.

This is also somewhat related to #2820.

Sebastian