Link: relationships expressed in a header

The header does what a link element does, without needing a document to put it in. That is the whole reason it exists: a PDF, a generated image or a JSON response has no head section, so the only place to express a relationship such as a canonical URL or a licence is in the headers.

For performance the interesting relation is preload, because a header is available to the browser before any of the body has been parsed. Pairing it with a 103 Early Hints response sends the preloads while the server is still assembling the real response, so the browser can start fetching stylesheets and fonts during time that would otherwise be idle.

API pagination is the other established pattern. Returning first, prev, next and last relations in this header keeps the page body free of navigation metadata and gives clients complete URLs to follow rather than parameters to assemble. Remember to expose the header for cross-origin callers, or the client sees nothing.

Several relations can be sent, either comma separated in one header or as repeated header lines. Both are valid, and a large preload list is usually easier to read as separate lines, though every line adds to the response header size on a path where that cost is paid before anything is rendered.