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enhance-web-instant-nav — Instant Navigations

Degree of freedom: MIXED. Which pages to speculate [HIGH freedom]; the exclusion list, Sec-Purpose handling, and re-measure [LOW freedom — run exactly].

Applies to MPAs and Next.js App Router (server-rendered routes). Pure SPA routing → use the framework's prefetch API; Speculation Rules are per-document.

How to reason

1. Observe — quote 2nd-page LCP p75, bfcache test result, and the top 5 next-click paths
2. Rank — probability × page cost. Prefetch broadly; prerender only ≥ ~50% likely next pages
3. Guard — list every URL with GET side effects or per-user state; exclude or downgrade to prefetch
4. Ship — rules + server guard + analytics guard in the same PR
5. Re-measure — if 2nd-page LCP didn't drop, the rule isn't firing (DevTools → Application → Speculative loads)

Worked example