Cache Manifest: What is the prefer-online setting?

Question

I've been looking for a way to cache pages for use only when the user is offline, otherwise download the pages normally. However, once a cache manifest is detected, the browser will only load from those cached pages.

My search landed me on http://whatwg.org. The section about Writing Cache Manifests caught my eye. Here's an excerpt:

A section header

Section headers change the current section. There are four possible section headers:

CACHE:
    Switches to the explicit section. 
FALLBACK:
    Switches to the fallback section. 
NETWORK:
    Switches to the online whitelist section. 
SETTINGS:
    Switches to the settings section.

The first three are well known, but I hadn't heard of the last section header, SETTINGS:. The documentation continues to say this:

Currently only one setting is defined:

The cache mode setting

This consists of the string "prefer-online". It sets the cache mode to prefer-online. (The cache mode defaults to fast.)

This made me wonder if the prefer-online setting was exactly what I was looking for. I was hoping that this setting would tell the browser that it should only load the cached pages if the setting was set to fast, otherwise download the pages if the user is online.

Unfortunately, I can't find any additional information or explanation about this. Has anyone come across any information about the prefer-online setting? Or about the SETTINGS: section in general?

Answer 1

prefer-online will go to the server if there is a connection, even if the file is cached. If there is no connection it will use the cache.

fast will use the offline version, even if there is a connection.

Although you can set these preferences, the user agent can still decide what it thinks is best.