Browser builds: the floor probe

With no native layer there is nothing to stop the bundle loading, so the package ships an ES5 probe to inline in <head>, before every other script: probe/floor-probe.js.

In a Tauri app on Android or iOS you do not need this

The plugin's native guard refuses to let the WebView load at all. The probe is for the plain browser build, and as a cross-check.

What it tests

The two features the floor is actually made of, not a version string:

Feature	Chromium	Safari	Firefox
color-mix()	111	16.2	113
@property	85	16.4	128

The floor (Chromium 111 / Safari 16.4 / Firefox 128) is the union of those two rows, so the gate is their conjunction: color-mix() andCSS.registerProperty as the @property sentinel. A color-mix()-only check would wave iOS 16.2 and 16.3 through. CSS.registerProperty stands in for @property because CSS.supports cannot test an at-rule, and the JS API and the at-rule shipped together in every engine.

What it does

• Always: sets <html data-webview-floor="supported|below"> and window.__WEBVIEW_GUARD_FLOOR__ = { supported, colorMix, registerProperty, computedColorMix }. computedColorMix is diagnostic only, never part of the verdict: an engine that drops the declaration reports the #123456 fallback, rgb(18, 52, 86).
• Supported: promotes every <script type="text/plain" data-webview-guard-src="…"> to a module script (keeping its crossorigin) — so a bundle parked that way is never handed to an engine that cannot parse it.
• Below: on DOMContentLoaded, replaces <body> with the content of your <template id="webview-guard-below-floor">, or a plain English "Update your browser" message if the page has no such template.

Inlining it

It must be inlined, not imported: an import is part of the bundle, and the bundle is exactly what a below-floor engine may fail to parse. With Vite, as the example app does:

import { readFileSync } from "node:fs";
import { createRequire } from "node:module";

import { defineConfig } from "vite";

const require = createRequire(import.meta.url);
const probe = readFileSync(
  require.resolve("tauri-plugin-webview-guard-api/probe/floor-probe.js"),
  "utf8",
);

export default defineConfig({
  plugins: [
    {
      name: "inline-webview-guard-floor-probe",
      transformIndexHtml: {
        order: "pre",
        handler: (html) =>
          html.replace("<!-- webview-guard:floor-probe -->", () => `<script>${probe}</script>`),
      },
    },
  ],
});

with the marker comment in index.html's <head>, before every other script:

<head>
  <meta charset="UTF-8" />
  <!-- webview-guard:floor-probe -->
  <script type="module" src="/src/main.ts"></script>
</head>

Park the bundle, or check the verdict

A Vite module script is not parked by default, and runs on Chromium 61–110 anyway — park it, or have your entry return early when document.documentElement.dataset.webviewFloor === 'below'.

How it is checked

Verified on Chromium 53, 109, 113 and 134, where its verdict agreed with the native one on every arm (the table); the parser test (acorn, ecmaVersion: 5) keeps it ES5.