On https://panopticlick.eff.org/ EFF let you test the number of uniquely identifying bits that the browser gives a website. Among these are HTTP header fields such as User-Agent, Accept, Accept-Language and later perhaps ETAG and If-Modified-Since. Also there is a lot of Information that javascript can get from the browser such as time-zone, screen resolution, complete list of fonts and plugins available.

My first impression is, is all this information really usable/used on a majority of all websites? For example, how many sites does really send different content-types depending on the http accept header, or what fonts are available(I thought css had taken care of this)?

Let's say of these headers/js functionality on day would be gone.
Which ones would;

• never be noticed they were gone?
• impact user experience?
• impact server performance?
• immediately reimplemented because the Internet cannot work without it?

Extra credit for differentiating between what can be done, what should be done and what is done in most situations.