Why browser does not send “If-None-Match” header?

Question

I\'m trying to download (and hopefully cache) a dynamically loaded image in PHP. Here are the headers sent and received:

Request:

GET /url:resource/P         


        

Answer 1

This was happening to me because I had set the cache size to be too small (via Group Policy).

It didn't happen in Incognito, which is what made me realize this might be the case.

Fixing that resolved the issue.