Navigation: instant without a client router

The feature#

June ships no client router — and navigation is still instant, because the browser already has one. Every HTML page carries three things:

1. Speculation Rules. A <script type="speculationrules"> block tells the browser to prerender same-origin links on hover (eagerness: moderate). By the time you click, the next page is already rendered — the click is a zero-network activation.
2. A fallback for the rest. Browsers without Speculation Rules get a tiny inline script that turns pointerover into <link rel="prefetch"> — progressive enhancement, not a polyfill bundle.
3. View Transitions. @view-transition { navigation: auto } animates the document swap (and respects prefers-reduced-motion). Cross-document navigation stops looking like a full reload because the browser paints it as one continuous surface. It is pure CSS — no JavaScript, no document.startViewTransition() (that API is for same-document SPAs) — and it requires both the outgoing and incoming page to carry the opt-in, on a same-origin navigation. June emits the rule on every page, so both ends always qualify.

The agent surfaces are deliberately excluded: .md, .json, and /mcp never prerender — machines don't hover.

Try it right now#

This page is the demo. Hover any link in the sidebar, watch the network panel prerender it, then click: no fetch, one smooth transition. Everything you just experienced shipped as ~1KB of declarative rules — not a router, not a virtual DOM, not a client cache to invalidate.

The posture#

Client routers exist to make navigation fast and transitions smooth. The platform now does both natively, so June's default is the same choice it makes everywhere (Standards): use the browser's mechanism and ship nothing. This is also why a full document — not a Flight payload (RSC) — is what crosses the wire: it covers most sites with zero client state to manage.

When an app-like surface genuinely needs in-memory state to survive a navigation (a dashboard, an open websocket), the opt-in Client Router adds soft swaps as progressive enhancement — off by default, and this site keeps it off.

Configured by speculation in june.config.ts, on by default — the config exists to turn it off.

The fine print#

This default leans on a few browser behaviors. They are load-bearing and easy to get subtly wrong, so they are stated here rather than left implicit:

• Back/forward is instant via bfcache — the browser restores a live page snapshot, no re-render. But a page served with Cache-Control: no-store is disqualified from bfcache in Chrome and Firefox, so don't reach for no-store on HTML to force revalidation; use no-cache or a short max-age. (Chrome is trialing a conditional relaxation — verify against your targets; behavior is in flux as of 2026-06.)
• Speculative requests are detectable server-side — a prerender carries Sec-Purpose: prefetch;prerender (a prefetch, just Sec-Purpose: prefetch), so a route can skip side effects until the page is actually activated.
• Hashed assets are served immutable (the worker sets it), so the browser skips even the conditional revalidation request within the asset's lifetime — the per-navigation cost stays at the document, not its CSS/JS.

Why it matters#

A client router is the single biggest source of accidental JavaScript: the router pulls in state, the state pulls in hydration, and suddenly a document site ships a runtime. Letting the browser navigate keeps the zero-JS default (Islands) honest.