link - rel : preconnect | prefetch | preload

The required rel attribute specifies the relationship between the current document and the linked document/resource.

• Specifies that the browser should preemptively connect to the target resource's origin. (preconnect)
• Specifies that the browser should preemptively fetch and cache the target resource as it is likely to be required for a follow-up navigation (prefetch)
• Specifies that the browser agent must preemptively fetch and cache the target resource for current navigation according to the destination given by the "as" attribute (and the priority associated with that destination). (preload)