diff --git a/web/templates/pwa/service-worker.js b/web/templates/pwa/service-worker.js
new file mode 100644
index 0000000..3ae7e9a
--- /dev/null
+++ b/web/templates/pwa/service-worker.js
@@ -0,0 +1,76 @@
+// Service Worker — Illustrations DB
+const VERSION = 'v1.0.0';
+const STATIC_CACHE = `static-${VERSION}`;
+const PAGES_CACHE  = `pages-${VERSION}`;
+const OFFLINE_URL  = '/offline/';
+
+const STATIC_ASSETS = [
+  '/',                       // optional: remove if you don’t want to cache /
+  '/static/pwa/icon-192.png',
+  '/static/pwa/icon-512.png'
+];
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open(STATIC_CACHE).then((cache) => cache.addAll(STATIC_ASSETS))
+      .then(() => self.skipWaiting())
+  );
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    caches.keys().then((keys) =>
+      Promise.all(
+        keys.filter(k => ![STATIC_CACHE, PAGES_CACHE].includes(k))
+            .map(k => caches.delete(k))
+      )
+    ).then(() => self.clients.claim())
+  );
+});
+
+function isNavigation(req) {
+  return req.mode === 'navigate' ||
+         (req.method === 'GET' && req.headers.get('accept')?.includes('text/html'));
+}
+
+self.addEventListener('fetch', (event) => {
+  const req = event.request;
+  if (req.method !== 'GET') return;
+
+  // HTML pages: stale-while-revalidate w/ offline fallback
+  if (isNavigation(req)) {
+    event.respondWith((async () => {
+      try {
+        const network = await fetch(req);
+        const cache = await caches.open(PAGES_CACHE);
+        cache.put(req, network.clone());
+        return network;
+      } catch {
+        const cache = await caches.open(PAGES_CACHE);
+        const cached = await cache.match(req);
+        return cached || caches.match(OFFLINE_URL);
+      }
+    })());
+    return;
+  }
+
+  // Static files under /static: cache-first
+  const url = new URL(req.url);
+  if (url.pathname.startsWith('/static/')) {
+    event.respondWith((async () => {
+      const cache = await caches.open(STATIC_CACHE);
+      const cached = await cache.match(req);
+      if (cached) return cached;
+      try {
+        const network = await fetch(req);
+        cache.put(req, network.clone());
+        return network;
+      } catch {
+        return cached; // best effort
+      }
+    })());
+    return;
+  }
+
+  // Default: pass-through
+});
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