It’s a best practice to reference static resources by using the $Resource global variable and the URLFOR() function instead of static strings. For example: These references are safe to use regardless of your organization’s settings for this Critical Update. We recommend that you test this update in a sandbox or Developer Edition organization to verify correct behavior of your pages before enabling it in your production organization.
To activate this critical update: From Setup, click Critical Updates. Click Activate for “Serve static resources from Visualforce Domain.”