What is a PWA, really — and how to tell if a site is one

Short answer

A Progressive Web App is a normal website with two additions: a manifest file that tells the browser how to install it, and a service worker that caches it so it keeps running with no connection. That is the whole definition. In practice it means a site you can add to your dock or home screen, that opens in its own window with no address bar, and that works on a train. No app store, no download, no review process — the browser does the installing.

The two pieces that make a PWA

The manifest is a small JSON file listing the app's name, its icons, and how it should open — usually standalone, meaning its own window rather than a browser tab. Without it, the browser has nothing to install.

The service worker is a script that sits between the page and the network. It stores the app's files the first time you visit, then serves them from that store afterwards. This is what makes a site open instantly on the second visit, and what makes it open at all when you are offline.

Everything else people associate with PWAs — push notifications, background sync, file handling — is optional. A site with a manifest and a service worker is a PWA. A site with neither is a website, however good it is.

Mentioned here: ✏️ Excalidraw, 🌀 Wormhole, 🖼️ Photopea

How to check if a site is one

The fastest test: open it in Chrome or Edge and look at the right end of the address bar. If an install icon appears, the site qualifies. No icon, no manifest — or you are in a private window, where installation is disabled.

The thorough test: open developer tools, go to the Application tab, and look at Manifest and Service Workers. You will see the declared name, the icons, the display mode, and whether a worker is registered and active.

The honest test: install it, turn off your connection, and open it. That is the only check that cannot be faked by a well-formed manifest — and it is the one we run on every listing in the directory.

Why any of this matters to you

No app store means no gatekeeper: nothing to approve, nothing to remove, no 30 % cut. It also means updates are instant — the version you open is always current, because there is no installed binary to be out of date.

It means one app across every device. The same whiteboard on your laptop, your phone and a borrowed machine, with no per-platform build.

And it means a much smaller footprint. A PWA stores a few megabytes of code and cached assets, where an equivalent native app often runs to hundreds.

Frequently asked

Are PWAs safe?

They run in the browser's sandbox, with the same permission model as any website — camera, location and notifications all require an explicit prompt. That is a tighter default than most installed applications.

Do PWAs work on iPhone?

Yes, through Safari — Share, then Add to Home Screen. Apple restricts some capabilities compared to Android, but installation, standalone windows and offline caching all work.