What requests do browsers' "F5" and "Ctrl + F5" refreshes generate?

Question

Is there a standard for what actions F5 and Ctrl+F5 trigger in web browsers?

I once did experiment in IE6 and Firefox 2.x. The F5 refresh would trigger a HTTP request sent to the server with an If-Modified-Since header, while Ctrl+F5 would not have such a header. In my understanding, F5 will try to utilize cached content as much as possible, while Ctrl+F5 is intended to abandon all cached content and just retrieve all content from the servers again.

But today, I noticed that in some of the latest browsers (Chrome, IE8) it doesn't work in this way anymore. Both F5 and Ctrl+F5 send the If-Modified-Since header.

So how is this supposed to work, or (if there is no standard) how do the major browsers differ in how they implement these refresh features?

Answer 1

It is up to the browser but they behave in similar ways.

F5 usually updates the page only if it is modified. Modern browsers sends Cache-Control: max-age=0 to tell any cache the maximum amount of time a resource is considered fresh, relative to the time of the request.

CTRL-F5 is used to force an update, disregarding any cache. Modern browsers sends Cache-Control: no-cache and Pragma: No-cache

If I remember correctly it was Netscape which was the first browser to add support for cache-control by adding Pragma: No-cache when you pressed CTRL-F5.

┌───────────┬──────────────┬─────┬─────────────────┬──────────────────────────────┐ │ Version 4 │      F5      │  R  │      CLICK      │ Legend:                      │ │2021 MAY 19├──┬──┬──┬──┬──┼──┬──┼──┬──┬──┬──┬──┬──┤ C = Cache-Control: no-cache  │ │           │  │S │C │A │A │C │C │  │S │C │A │A │C │ I = If-Modified-Since        │ │           │  │H │T │L │L │T │T │  │H │T │L │L │T │ M = Cache-Control: max-age=0 │ │           │  │I │R │T │T │R │R │  │I │R │T │T │R │ N = Not tested               │ │           │  │F │L │  │G │L │L │  │F │L │  │G │L │ P = Pragma: No-cache         │ │           │  │T │  │  │R │  │+ │  │T │  │  │R │+ │ - = ignored                  │ │           │  │  │  │  │  │  │S │  │  │  │  │  │S │                              │ │           │  │  │  │  │  │  │H │  │  │  │  │  │H │ With 'CLICK' I refer to a    │ │           │  │  │  │  │  │  │I │  │  │  │  │  │I │ mouse click on the browsers  │ │           │  │  │  │  │  │  │F │  │  │  │  │  │F │ refresh-icon.                │ │           │  │  │  │  │  │  │T │  │  │  │  │  │T │                              │ │           │  │  │  │  │  │  │  │  │  │  │  │  │  │ 1: Version 3.0.6 sends I     │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤    and C, but 3.1.6 opens    │ │Brave 1.24 │M │CP│CP│- │- │M │CP│M │CP│CP│M │CP│CP│    the page in a new tab,    │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤    making a normal request   │ │Chrome 1   │MI│MI│MI│- │- │MI│- │MI│MI│MI│MI│MI│N │    with only I.              │ │Chrome 6   │MI│CP│CP│- │- │MI│CP│MI│CP│CP│MI│- │N │ 2: Version 10.62 does        │ │Chrome 90  │M │CP│CP│- │- │M │CP│M │CP│CP│M │CP│CP│    nothing. 9.61 might do C  │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤    unless it was a typo in   │ │Edge 90    │M │CP│CP│- │- │M │CP│M │CP│CP│M │CP│CP│    my old table.             │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤ 3: Opens the currernt tab in │ │Firefox 3.x│MI│- │CP│- │- │MI│CP│MI│CP│1 │M │MI│N │    a new tab, but does not   │ │Firefox 89 │M │- │CP│- │M │M │CP│M │CP│3 │M │M │3 │    refresh the page if it is │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤    cached in the browser.    │ │MSIE 8, 7  │I │- │C │- │I │I │  │I │I │C │I │I │N │                              │ ├───────────┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┼──┤                              │ │Opera 10, 9│C │- │- │2 │- │C │- │C │C │C │C │- │N │                              │ │Opera 76   │M │CP│CP│- │- │M │- │M │CP│CP│M │CP│CP│                              │ ├───────────┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──────────────────────────────┤ │                    https://stackoverflow.com/a/385491/36866                     │ └─────────────────────────────────────────────────────────────────────────────────┘ 

Note about Chrome 6.0.472: If you do a forced reload (like CTRL-F5) it behaves like the url is internally marked to always do a forced reload. The flag is cleared if you go to the address bar and press enter.

Answer 2

Generally speaking:

F5 may give you the same page even if the content is changed, because it may load the page from cache. But Ctrl+F5 forces a cache refresh, and will guarantee that if the content is changed, you will get the new content.