Page Speed: Leverage browser caching

Page Speed recommends:

Set caching headers aggressively for all static resources

For all cacheable resources, we recommend the following settings:

• Set Expires to a minimum of one month, and preferably up to one year, in the future. (We prefer Expires over Cache-Control: max-age because it is is more widely supported.) Do not set it to more than one year in the future, as that violates the RFC guidelines.
• Set the Last-Modified date to the last time the resource was changed. If the Last-Modified date is sufficiently far enough in the past, chances are the browser won't refetch it.

Use fingerprinting to dynamically enable caching

For resources that change occasionally, you can have the browser cache the resource until it changes on the server, at which point the server tells the browser that a new version is available. You accomplish this by embedding a fingerprint of the resource in its URL (i.e. the file path).

Set the Vary header correctly for Internet Explorer

Internet Explorer does not cache any resources that are served with the Vary header and any fields but Accept-Encoding and User-Agent. To ensure these resources are cached by IE, make sure to strip out any other fields from the Vary header, or remove the Vary header altogether if possible

Avoid URLs that cause cache collisions in Firefox

The Firefox disk cache hash functions can generate collisions for URLs that differ only slightly, namely only on 8-character boundaries.

Use the Cache control: public directive to enable HTTPS caching for Firefox

Some versions of Firefox require that the Cache control: public header to be set in order for resources sent over SSL to be cached on disk, even if the other caching headers are explicitly set. Although this header is normally used to enable caching by proxy servers (as described below), proxies cannot cache any content sent over HTTPS, so it is always safe to set this header for HTTPS resources.

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