My first impression is, is all this information really used/required 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.