diff --git a/CLAUDE.md b/CLAUDE.md
index aa0a71d..d47d814 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -126,7 +126,7 @@ mailboxzero/
   - JSON API responses
   - Error handling and logging
   - Static file serving
-  - `Handler()` builds the router behind the request checks described under Security Considerations (testable end to end); `Start()` serves it through `newHTTPServer`, which sets timeouts: 10 s to read headers, 30 s to read the request, 2 min to write the response (Find Similar fetches up to 10,000 emails in JMAP requests of up to 1000, each with a 30 s client timeout, then groups them), 2 min idle
+  - `Handler()` builds the router behind the request checks and response headers described under Security Considerations (testable end to end); `Start()` serves it through `newHTTPServer`, which sets timeouts: 10 s to read headers, 30 s to read the request, 2 min to write the response (Find Similar fetches up to 10,000 emails in JMAP requests of up to 1000, each with a 30 s client timeout, then groups them), 2 min idle
   - Find Similar searches the newest 10,000 inbox emails (`maxSimilarityEmails`), which the client fetches in pages of up to 1000 (or the server's `maxObjectsInGet`). A targeted email outside them (older, or newer than the cache) is fetched by itself with `GetEmail`
   - Inbox cache for `/api/similar`: that fetch is cached in-process for 60 seconds (`inboxCacheTTL`). A successful archive removes the archived emails from the cache (a dry run leaves it), so successive "Find Similar" / "Archive & Find Next" calls don't re-pull from JMAP; a failed archive (JMAP can reject some emails while moving the rest) and any unarchive invalidate it
 
@@ -190,7 +190,7 @@ default_similarity: 60          # Default similarity percentage (0-100)
 
 ### Security Considerations
 - **Localhost Only:** The web UI has no login, so `validate()` in `internal/config/config.go` rejects any `server.host` that `IsLoopbackHost` does not accept (`localhost`, 127.0.0.0/8, `::1`), including empty and `0.0.0.0`. The app deliberately provides no authentication or HTTPS; remote use goes through an SSH tunnel
-- **Browser Protections:** `Handler()` wraps the router in `localhostOnly` (403 unless the `Host` header is a loopback name, which stops DNS rebinding) and Go's `http.CrossOriginProtection` (403 for cross-origin `POST`s via `Sec-Fetch-Site`/`Origin`, which stops cross-site request forgery). `e2e/tests/cross-site.spec.js` checks the latter in a real browser
+- **Browser Protections:** `Handler()` wraps the router in `localhostOnly` (403 unless the `Host` header is a loopback name, which stops DNS rebinding) and Go's `http.CrossOriginProtection` (403 for cross-origin `POST`s via `Sec-Fetch-Site`/`Origin`, which stops cross-site request forgery). Outside both, `securityHeaders` sets on every response, refusals included, the Content-Security-Policy `contentSecurityPolicy` (only the app's own script, stylesheet, images and API; no inline script, `on…` handler attributes or `style` attributes; `frame-ancestors 'none'`), `X-Frame-Options: DENY` and `X-Content-Type-Options: nosniff`. That stops other sites from framing the page to trick clicks or keys such as `A` (clickjacking), and stops any markup that slips into the page from running or loading anything. So style elements with classes or the `hidden` attribute, never `style` attributes (setting `element.style` from JavaScript is allowed). `e2e/tests/cross-site.spec.js` checks the forgery and framing protections in a real browser, and `e2e/tests/content-security-policy.spec.js` checks that injected script does not run and that the app works with no policy violations
 - **API Tokens:** Use Fastmail API tokens for secure authentication
 - **Local Processing:** All similarity calculations happen locally
 - **Minimal Permissions:** Uses only the JMAP Core and Mail capabilities (`urn:ietf:params:jmap:mail`), and its only changes move emails between the inbox and the archive
diff --git a/README.md b/README.md
index 5a1c856..086385a 100644
--- a/README.md
+++ b/README.md
@@ -171,7 +171,7 @@ To stay fast on large inboxes, grouping only compares emails that could reach th
 ## Security Considerations
 
 - **Localhost Only**: The web UI has no login, so the server only listens on this machine. `server.host` must be `localhost`, `127.0.0.1` or `::1`; anything else (including `0.0.0.0`) is rejected at startup. To use it from another machine, forward the port over SSH (`ssh -L 8080:localhost:8080 your-server`) and open http://localhost:8080 locally
-- **Browser Protections**: Other websites open in your browser cannot use the app: requests addressed to any host name but localhost are refused (blocking DNS rebinding), and cross-origin `POST` requests are refused (blocking cross-site request forgery)
+- **Browser Protections**: Other websites open in your browser cannot use the app: requests addressed to any host name but localhost are refused (blocking DNS rebinding), and cross-origin `POST` requests are refused (blocking cross-site request forgery). The app also refuses to be shown in another site's frame (blocking clickjacking), and its Content-Security-Policy lets the page run only its own scripts and styles
 - **API Tokens**: Use Fastmail API tokens for secure authentication
 - **Local Only**: All processing happens locally; the server sends requests only to Fastmail's JMAP API
 - **Read-Heavy**: Only reads email data, minimal write operations
diff --git a/e2e/tests/content-security-policy.spec.js b/e2e/tests/content-security-policy.spec.js
new file mode 100644
index 0000000..af9b5ad
--- /dev/null
+++ b/e2e/tests/content-security-policy.spec.js
@@ -0,0 +1,76 @@
+// @ts-check
+const { test, expect } = require('@playwright/test');
+
+// Record every Content-Security-Policy violation the page reports.
+test.beforeEach(async ({ page }) => {
+  await page.addInitScript(() => {
+    const violations = [];
+    /** @type {any} */ (window).cspViolations = violations;
+    document.addEventListener('securitypolicyviolation', (e) => {
+      violations.push({ directive: e.effectiveDirective, blocked: e.blockedURI });
+    });
+  });
+});
+
+const violations = (page) => page.evaluate(() => /** @type {any} */ (window).cspViolations);
+
+test('the page runs no script injected into it', async ({ page }) => {
+  await page.goto('/');
+
+  // Markup that slipped into the page, the way a bug in the preview once
+  // let an email's HTML do: an image whose error handler runs script.
+  await page.evaluate(() => new Promise((resolve) => {
+    const image = document.createElement('img');
+    image.setAttribute('onerror', 'window.injectedHandlerRan = true');
+    image.addEventListener('error', resolve);
+    image.src = '/no-such-image.png';
+    document.body.appendChild(image);
+  }));
+
+  expect(await page.evaluate(() => /** @type {any} */ (window).injectedHandlerRan)).toBeUndefined();
+  await expect.poll(() => violations(page)).toContainEqual({ directive: 'script-src-attr', blocked: 'inline' });
+});
+
+test('the app works within its own policy', async ({ page }) => {
+  const response = await page.goto('/');
+  expect(response?.headers()['content-security-policy']).toContain("default-src 'none'");
+  await expect(page.locator('#inbox-list .email-item').first()).toBeVisible();
+  await expect(page.locator('#archive-status')).toBeHidden();
+
+  await page.click('#find-similar-btn');
+  const similar = page.locator('#similar-list .email-item').first();
+  await expect(similar).toBeVisible();
+
+  // A preview marks up links and addresses with classes, not style
+  // attributes, which the policy would block.
+  const id = await similar.getAttribute('data-email-id');
+  await page.route((url) => url.pathname === `/api/emails/${id}`, (route) => route.fulfill({
+    json: {
+      bodyValues: { 1: { value: 'See https://example.com/offer or write to help@example.com' } },
+      textBody: [{ partId: '1', type: 'text/plain' }],
+    },
+  }));
+  await similar.hover();
+  const link = page.locator('#preview-body span', { hasText: 'https://example.com/offer' });
+  await expect(link).toHaveCSS('color', 'rgb(52, 152, 219)');
+  await expect(link).toHaveCSS('text-decoration-line', 'underline');
+  await expect(page.locator('#preview-body span', { hasText: 'help@example.com' })).toHaveCSS('color', 'rgb(52, 152, 219)');
+  await page.keyboard.press('Escape');
+
+  // The status pill shows once something is archived. Undo puts the emails
+  // back, since every test shares the mock inbox.
+  await page.click('#archive-find-next-btn');
+  await expect(page.locator('#archive-status')).toBeVisible();
+  await expect(page.locator('#archive-status-text')).toHaveText(/^Successfully archived/);
+  await page.click('#archive-undo-btn');
+  await expect(page.locator('#archive-status-text')).toHaveText(/^Unarchived/);
+
+  // An error message keeps its color.
+  await page.route((url) => url.pathname === '/api/similar', (route) => route.fulfill({ status: 500, body: 'failed' }));
+  await page.click('#find-similar-btn');
+  const error = page.locator('#similar-list .empty-state p');
+  await expect(error).toHaveText('Failed to find similar emails.');
+  await expect(error).toHaveCSS('color', 'rgb(231, 76, 60)');
+
+  expect(await violations(page)).toEqual([]);
+});
diff --git a/e2e/tests/cross-site.spec.js b/e2e/tests/cross-site.spec.js
index 408b73f..424f1d1 100644
--- a/e2e/tests/cross-site.spec.js
+++ b/e2e/tests/cross-site.spec.js
@@ -42,3 +42,24 @@ test('a page on another site cannot archive emails through the browser', async (
     await new Promise((resolve) => attacker.close(resolve));
   }
 });
+
+// A site that could show the app in a frame (say, invisibly over its own
+// buttons) could trick the user into clicking Archive & Find Next.
+test('a page on another site cannot show the app in a frame', async ({ page, baseURL }) => {
+  const attacker = http.createServer((req, res) => {
+    res.setHeader('Content-Type', 'text/html');
+    res.end(`<!doctype html><title>Some other site</title><iframe src="${baseURL}/"></iframe>`);
+  });
+  await new Promise((resolve) => attacker.listen(attackerPort, '127.0.0.1', resolve));
+
+  try {
+    // The browser does ask the app for the page; it just must not show it.
+    const appRequest = page.waitForRequest(`${baseURL}/`);
+    await page.goto(`http://127.0.0.1:${attackerPort}/`);
+    await appRequest;
+
+    await expect(page.frameLocator('iframe').locator('#inbox-list')).toHaveCount(0);
+  } finally {
+    await new Promise((resolve) => attacker.close(resolve));
+  }
+});
diff --git a/internal/server/server.go b/internal/server/server.go
index 7ff0a94..89a7236 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -45,6 +45,14 @@ const (
 	idleTimeout       = 2 * time.Minute
 )
 
+// contentSecurityPolicy lets the page use only its own script, stylesheet,
+// images and API, with no inline script, event handler attributes or style
+// attributes, so markup that ever slips into the page can neither run nor
+// load anything. frame-ancestors keeps other sites from showing the page in
+// a frame. img-src allows the browser's own request for /favicon.ico.
+const contentSecurityPolicy = "default-src 'none'; script-src 'self'; style-src 'self'; img-src 'self'; " +
+	"connect-src 'self'; base-uri 'none'; form-action 'none'; frame-ancestors 'none'"
+
 type Server struct {
 	config     *config.Config
 	jmapClient jmap.JMAPClient
@@ -84,8 +92,10 @@ func (s *Server) Start() error {
 // Handler returns the HTTP handler that serves the UI, static files and API.
 //
 // Listening on localhost only keeps other machines out, but every website
-// open in the user's browser can still send requests here. Two checks run
-// before the router:
+// open in the user's browser can still send requests here, or show the page
+// in a frame. Three layers run before the router:
+//   - securityHeaders tells the browser, on every response (refusals too),
+//     what the page may load and run, and not to show it in a frame.
 //   - localhostOnly refuses requests addressed to any other host name, which
 //     stops DNS rebinding (a foreign domain re-pointed at 127.0.0.1).
 //   - CrossOriginProtection refuses cross-origin POSTs, so another site cannot
@@ -103,7 +113,20 @@ func (s *Server) Handler() http.Handler {
 	r.HandleFunc("/api/unarchive", s.handleUnarchive).Methods("POST")
 	r.HandleFunc("/api/clear", s.handleClear).Methods("POST")
 
-	return localhostOnly(http.NewCrossOriginProtection().Handler(r))
+	return securityHeaders(localhostOnly(http.NewCrossOriginProtection().Handler(r)))
+}
+
+// securityHeaders sets the Content-Security-Policy, refuses framing (also
+// for browsers that only know X-Frame-Options) and stops the browser from
+// guessing a response's type, so a response is never run as something else.
+func securityHeaders(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("Content-Security-Policy", contentSecurityPolicy)
+		h.Set("X-Frame-Options", "DENY")
+		h.Set("X-Content-Type-Options", "nosniff")
+		next.ServeHTTP(w, r)
+	})
 }
 
 // localhostOnly serves only requests whose Host header names this machine.
diff --git a/internal/server/server_test.go b/internal/server/server_test.go
index f677b5d..bc21aa0 100644
--- a/internal/server/server_test.go
+++ b/internal/server/server_test.go
@@ -1532,3 +1532,68 @@ func TestHandleGetEmail(t *testing.T) {
 		})
 	}
 }
+
+// Every response, including refusals, tells the browser to run only the
+// app's own scripts and styles and never to show the page in a frame.
+func TestHandler_SecurityHeaders(t *testing.T) {
+	server := setupTestServer(t)
+	handler := server.Handler()
+
+	tests := []struct {
+		name     string
+		method   string
+		path     string
+		host     string
+		headers  map[string]string
+		want     int
+		wantType string // a part of Content-Type that nosniff relies on
+	}{
+		{name: "page", method: "GET", path: "/", want: http.StatusOK, wantType: "text/html"},
+		{name: "script", method: "GET", path: "/static/app.js", want: http.StatusOK, wantType: "javascript"},
+		{name: "stylesheet", method: "GET", path: "/static/style.css", want: http.StatusOK, wantType: "text/css"},
+		{name: "API", method: "GET", path: "/api/emails", want: http.StatusOK, wantType: "application/json"},
+		{name: "not found", method: "GET", path: "/no-such-page", want: http.StatusNotFound},
+		{name: "foreign host", method: "GET", path: "/", host: "evil.example:8080", want: http.StatusForbidden},
+		{
+			name: "cross-site POST", method: "POST", path: "/api/clear",
+			headers: map[string]string{"Sec-Fetch-Site": "cross-site"}, want: http.StatusForbidden,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := httptest.NewRequest(tt.method, tt.path, nil)
+			req.Host = "localhost:8080"
+			if tt.host != "" {
+				req.Host = tt.host
+			}
+			for k, v := range tt.headers {
+				req.Header.Set(k, v)
+			}
+			w := httptest.NewRecorder()
+			handler.ServeHTTP(w, req)
+
+			if w.Code != tt.want {
+				t.Errorf("status = %d, want %d", w.Code, tt.want)
+			}
+			if got := w.Header().Get("X-Frame-Options"); got != "DENY" {
+				t.Errorf("X-Frame-Options = %q, want DENY", got)
+			}
+			if got := w.Header().Get("X-Content-Type-Options"); got != "nosniff" {
+				t.Errorf("X-Content-Type-Options = %q, want nosniff", got)
+			}
+			var directives []string
+			for _, directive := range strings.Split(w.Header().Get("Content-Security-Policy"), ";") {
+				directives = append(directives, strings.TrimSpace(directive))
+			}
+			for _, want := range []string{"default-src 'none'", "script-src 'self'", "style-src 'self'", "frame-ancestors 'none'"} {
+				if !slices.Contains(directives, want) {
+					t.Errorf("Content-Security-Policy %q lacks %q", w.Header().Get("Content-Security-Policy"), want)
+				}
+			}
+			if got := w.Header().Get("Content-Type"); !strings.Contains(got, tt.wantType) {
+				t.Errorf("Content-Type = %q, want it to contain %q", got, tt.wantType)
+			}
+		})
+	}
+}
diff --git a/web/static/app.js b/web/static/app.js
index affe6e3..7cea9c6 100644
--- a/web/static/app.js
+++ b/web/static/app.js
@@ -348,7 +348,7 @@ class MailboxZero {
         this.archiveStatus.classList.remove('error');
         this.archiveStatus.classList.toggle('dry-run', Boolean(dryRun));
         this.archiveUndoBtn.style.display = dryRun ? 'none' : '';
-        this.archiveStatus.style.display = '';
+        this.archiveStatus.hidden = false;
     }
 
     // Show a failed archive or undo in the status pill, with the reason,
@@ -359,7 +359,7 @@ class MailboxZero {
         this.archiveStatus.classList.remove('dry-run');
         this.archiveStatus.classList.add('error');
         this.archiveUndoBtn.style.display = 'none';
-        this.archiveStatus.style.display = '';
+        this.archiveStatus.hidden = false;
     }
 
     // Turn a failed response into an Error carrying the server's message,
@@ -795,7 +795,7 @@ class MailboxZero {
     }
 
     showError(container, message) {
-        container.innerHTML = `<div class="empty-state"><p style="color: #e74c3c;">${message}</p></div>`;
+        container.innerHTML = `<div class="empty-state"><p class="error-message">${message}</p></div>`;
     }
 
     showEmpty(container, message) {
@@ -1047,11 +1047,11 @@ class MailboxZero {
         
         // Format URLs (make them look like links without actually being clickable)
         formatted = formatted.replace(/\b(https?:\/\/[\w\-\._~:\/?#\[\]@!\$&'\(\)*\+,;=.]+)/gi, 
-            '<span style="color: #3498db; text-decoration: underline;">$1</span>');
+            '<span class="preview-link">$1</span>');
         
         // Format email addresses  
         formatted = formatted.replace(/\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b/g, 
-            '<span style="color: #3498db;">$&</span>');
+            '<span class="preview-address">$&</span>');
         
         // Bold text patterns (common email signatures)
         formatted = formatted.replace(/\*\*([^*]+)\*\*/g, '<strong>$1</strong>');
@@ -1059,7 +1059,7 @@ class MailboxZero {
         
         // Format common patterns like "From:", "To:", "Subject:" in forwarded emails
         formatted = formatted.replace(/^(From|To|Subject|Date|CC|BCC):\s*/gim, 
-            '<strong style="color: #666;">$1:</strong> ');
+            '<strong class="preview-label">$1:</strong> ');
         
         return formatted;
     }
diff --git a/web/static/style.css b/web/static/style.css
index 199be03..1b227a9 100644
--- a/web/static/style.css
+++ b/web/static/style.css
@@ -411,6 +411,12 @@ header h1 {
     border-radius: 5px;
 }
 
+/* The pill starts hidden; display: flex above would override the browser's
+   own rule for the hidden attribute. */
+.archive-status[hidden] {
+    display: none;
+}
+
 .archive-status-text {
     color: #1e7e34;
     font-size: 0.9em;
@@ -479,6 +485,10 @@ header h1 {
     font-size: 0.9em;
 }
 
+.empty-state .error-message {
+    color: #e74c3c;
+}
+
 .modal-overlay {
     display: none;
     position: fixed;
@@ -636,6 +646,22 @@ header h1 {
     color: #555;
 }
 
+/* Links, addresses and header labels marked up by enhanceEmailFormatting()
+   in app.js. They are classes, not style attributes, because the page's
+   Content-Security-Policy blocks style attributes. */
+.preview-body .preview-link {
+    color: #3498db;
+    text-decoration: underline;
+}
+
+.preview-body .preview-address {
+    color: #3498db;
+}
+
+.preview-body .preview-label {
+    color: #666;
+}
+
 .preview-loading {
     padding: 40px 20px;
     text-align: center;
diff --git a/web/templates/index.html b/web/templates/index.html
index 9d4beed..8c3b452 100644
--- a/web/templates/index.html
+++ b/web/templates/index.html
@@ -38,7 +38,7 @@ <h1>Mailbox Zero</h1>
                     </label>
                     <button id="archive-btn" class="btn btn-danger" disabled>Archive Selected</button>
                     <button id="archive-find-next-btn" class="btn btn-danger" title="Archive &amp; Find Next (A)" aria-keyshortcuts="A" disabled>Archive &amp; Find Next</button>
-                    <div id="archive-status" class="archive-status" style="display: none;">
+                    <div id="archive-status" class="archive-status" hidden>
                         <span id="archive-status-text" class="archive-status-text"></span>
                         <button id="archive-undo-btn" class="btn btn-secondary archive-status-undo" title="Undo (U)" aria-keyshortcuts="U">Undo</button>
                     </div>