# Browser compatibility

The distributed package targets ES2020 and is exercised end to end against the
three browser engines through Playwright on every pull request.

| Capability | Chromium | Firefox | WebKit | Fallback |
| --- | :---: | :---: | :---: | --- |
| Core signals and custom events | Tested | Tested | Tested | — |
| Fetch and XHR instrumentation | Tested | Tested | Tested | Missing APIs are skipped |
| Error and rejection capture | Tested | Tested | Tested | Missing browser globals are skipped |
| Web Vitals | Tested | Tested | Tested | Unsupported metrics remain `null` |
| Resource Timing | Tested | Tested | Tested | `renderBlocking` and `status` are `null` where unsupported |
| Web Vitals attribution | Tested | Feature detected | Feature detected | Fields the engine cannot measure are `null` |
| FPS | Tested | Tested | Tested | Requires `requestAnimationFrame` |
| JS heap memory | Supported | `null` | `null` | Chromium-only `performance.memory` |
| Page memory measurement | Feature detected | `null` | `null` | Chromium-only `measureUserAgentSpecificMemory()`, only in cross-origin isolated pages (COOP and COEP headers; see [Enabling page memory measurement](README.md#enabling-page-memory-measurement)) |
| Logical processor count | Supported | Supported | Supported | `null` where `navigator.hardwareConcurrency` is absent |
| Online status | Supported | Supported | Supported | `null` where `navigator.onLine` is absent |
| Browser name, version, platform | Client Hints | User-Agent string | User-Agent string | Unrecognized browsers report `null` name and version |
| Device memory | Supported | `null` | `null` | Chromium-only `navigator.deviceMemory` |
| Network quality | Supported | `null` | `null` | Chromium-only `navigator.connection` |
| Language, time zone, screen, viewport, color scheme, reduced motion | Supported | Supported | Supported | `null` where the API is absent |
| Long Tasks | Feature detected | Feature detected | Feature detected | Empty aggregate when unsupported |
| React commit collection | Feature detected | Feature detected | Feature detected | Requires the React DevTools global hook |
| Server-side import/start | Tested in Node | Tested in Node | Tested in Node | Browser collectors no-op |

“Tested” means the demo smoke flow loads the published build shape and validates
events, fetch, XHR, errors, reporting, and snapshots in Playwright's current
Chromium, Firefox, and WebKit revisions. Native metric availability still depends
on each engine. The collectors feature-detect optional platform APIs and retain
`null` or empty values when an API is absent.

Run the same matrix locally with:

```sh
npx playwright install chromium firefox webkit
npm run test:browser
```

Older browsers must support the ES2020 output and the APIs used by the enabled
collectors. The `ssignal` dependency also requires `WeakRef` and
`FinalizationRegistry` (ES2021: Chrome 84, Firefox 79, Safari 14.1). Declaring a
monitor with `using` additionally requires `Symbol.dispose`; where it is absent,
call `monitor.destroy()` instead. Applications that target older syntax should transpile the package
as part of their own build.