It's important to serve a resource from a unique URL, to eliminate duplicate download bytes and additional RTTs.

Guidelines:

Sometimes it's necessary to reference the same resource from multiple places in a page — images are a typical example. Even more likely is that you share the same resources across multiple pages in a site such as .css and .js files.

• For resources that are shared across multiple pages, make sure that each reference to the same resource uses an identical URL.


• If a resource is shared by multiple pages/sites that link to each other, but are hosted on different domains or hostnames, it's better to serve the file from a single hostname than to re-serve it from the hostname of each parent document. In this case, the caching benefits may outweigh the DNS lookup overhead.

Example:

For example, if both mysite.example.com and yoursite.example.com use the same JS file, and mysite.example.com links to yoursite.example.com (which will require a DNS lookup anyway), it makes sense to just serve the JS file from mysite.example.com. In this way, the file is likely to already be in the browser cache when the user goes to yoursite.example.com.