Why it exists

Tailwind v4's browser floor is Chromium 111 / Safari 16.4 / Firefox 128 — the union of color-mix() (Chromium 111, Safari 16.2) and @property (Chromium 85, Safari 16.4). Below it an app does not crash. It does something worse:

• Chromium 99–110 lays out perfectly and gets every derived colour wrong. Each color-mix(in oklab, var(--x) 30%, transparent) is invalid, so the token falls back to full strength: a 30 %-opacity skeleton becomes a near-white blob, a subtle border becomes a hard line. The app looks subtly wrong, and the report reads as a bug, not as an old engine.
• Below Chromium 99 there are no cascade layers, and Tailwind v4 emits its entire stylesheet inside @layer. A parser that does not know the at-rule discards the whole block: a white screen.

The owner of the apps this was built for has ruled the floor final — no normalizer, no polyfill, no transpilation. So the job is not to make an old engine work; it is to refuse to boot into one and say how to fix it, instead of showing a screen the user will read as "this app is broken".

On Android the fix is usually two taps: the WebView is a Play-updatable component, independent of the OS. On iOS WKWebView ships with the OS, so the real defence is the App Store refusing to install at all (see iOS: the deployment target is the real defence); the in-app check is the second line.

Why the dialog is native, never HTML

On Chromium 53, Tauri's own injected IPC script does not parse (object spread, Chromium 60), so window.__TAURI_INTERNALS__.postMessage is never defined and every invoke() rejects — measured, see the verification note. An HTML fallback page whose "Update" button calls a Tauri command is dead on exactly the devices it exists for. An AlertDialog / UIAlertController has no dependency on the engine it is judging.

Why the check is native, never a JS probe in the bundle

A bundle that cannot parse cannot run the check that explains why it cannot parse. The guard reads the engine version natively, during plugin setup — which Tauri runs before it creates the first WebView — and refuses the WebView's first navigation. The lifecycle has the sequence, with the Tauri and wry source lines behind each step.

For a build with no native layer at all — the plain browser build — the package ships an ES5 floor probe instead.