Currently developers can only specify speculation rules using inline script tags. The proposed feature provides an alternative through the "Speculation-Rules" header. Its value must be a URL to a text resource with "application/speculationrules+json" MIME type. The resource's rules will be added to the document's rule set.
To adopt speculation rule based preloading, the current approach of using the script tag to specify speculation rules requires modifying existing documents, which is more difficult for some authors to deploy. The proposed Speculation-Rule header provides an alternative to facilitate adoption of the speculation rule based preloading.
Explainers: https://github.com/WICG/nav-speculation/blob/main/triggers.md#the-proposal