This page was pre-fetched along with its image when the main page was loaded, so it could be pre-cached. This is a trick done to demonstrate that cached content is available offline, which is not typical of websites. Unbust Cache sets the caching headers specifically to allow static content to be available offline.
There are two image assets used on this page.
The left image should not be available if you navigated to this page without an internet connection.
The right image is prefetched from the top entry point, so it should be available offline. Normally assets that are essential for the layout/functionality of the site would be prefetched, while images that are "content", such as the image of books would not expected to be available offline.
Typical blog content (not pre-fetched)
Typical web design resources (pre-fetched)
The other two demos demonstrate more typical setups, where caching does not accomodate offline use because it requires the browser to re-validate the entry point on navigation.
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