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Deprecated!   Use Service Workers or Cache API instead.
Why service workers are better

Creating HTML5 Offline Web Applications
Common Appcache pitfalls also:

• mime type config in servers

Specifying a whole folder of files won't cache every file, only those loaded.

If clearing cache in Firefox doesn't work, then also clearing Site Data under Options -> Privacy & Security might work.

Access cache in Firefox: about:cache

Quirks

• cache clear doesn't always work at first (eg console might show that 'css' (folder) fetch failed, even tho the erroneous line was removed and cache cleared)
• while offline, browser (chrome, firefox) refresh might result in page load error yet if the page is opened in a new tab, it loads just fine - seems to result from a faulty cache
• any mistakes in appcache will halt it being read further (at least in firefox)
• maybe some problems have to do with keeping multiple tabs of the website open
• to force cache manifest to update, add or remove a comment
• after going offline, it can take a long time (15 seconds) for the cache to kick in...
• testing in localhost doesnt work