What is the 304 Not Modified HTTP status ?
When a client (intermediate / browser) can serve the content from its cache store, it may send a permission request (knwon as a conditional request) to the server. If the server responses back with the redirection status code 304, the cache gets the permission to use it.
In the devtool, a 304 shows that the browser has taken the decision to show the cached resource (and add therefore performed an implicit redirection)
Alongside a 304, the server must also add all cache control headers (as in a 200 (OK))
HTTP/1.1 304 Not Modified
You can see the status in the devtool
- or for a page load