Progressive Web App

The word “progressive” points to progressive enhancement: the same URL loads for every visitor, then browsers that support the extra APIs layer on the app-like behavior. Nothing about a PWA replaces plain HTML.

What makes a site a PWA

Two pieces do the work. A web app manifest is a JSON file that names the app and lists icons (install criteria expect a 192px and a 512px version), a start_url, and a display mode such as standalone. The manifest plus HTTPS is what makes a site installable. A service worker is a separate script that sits between the page and the network, caching responses so the app can open offline or on a flaky connection, and it is what powers offline use and push notifications. web.dev groups the payoff into three traits: capable, reliable, and installable.

The SEO catch is that a PWA is usually a JavaScript app, and Googlebot handles those in stages. It crawls the URL, queues the page for rendering in headless Chromium, then indexes the rendered DOM. That render step can lag, and content that only appears after JavaScript runs may be indexed late or not at all. Google’s own advice is server-side rendering or prerendering, so a full HTML page reaches crawlers (and users) on the first request. Use the History API for routing so every view has a real, crawlable URL instead of a #fragment, and return proper status codes so empty views do not become soft 404s. Googlebot renders each URL on its own, so every page must serve its own content, not just an empty app shell.