Making a user's browser cache files that rarely change is one of the best ways to speed up your website. Images (GIF, JPG, PNG, etc), CSS, and Javascript are the best candidated for caching as they rarely do not change. However, make sure you used versioning in your html (add a unix timestamp to the address of a .css file with file.css?1294857284, for example) to make sure that when your files change, usres are using the most recent version of the file.
File Extensions
Comma separated list. Omit leading "."
Cache Duration
Public Private
Must Revalidate

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