diff --git a/README.md b/README.md
index 6155baf..e97e9f7 100644
--- a/README.md
+++ b/README.md
@@ -189,7 +189,10 @@ red PR job against.
   `'unsafe-hashes'` without handlers is rejected. The meta must precede every
   `<script>`/`<link>`/`<img>`/`<style>` in `<head>`. Every external `<img>` origin must
   be allowed by `img-src` (or `default-src` when `img-src` is absent), either explicitly
-  or via `https:`. `base-uri` must contain `'none'` or `'self'`, and `form-action` must
+  or via `https:`. Every external `<iframe>` origin must likewise be allowed by
+  `frame-src` (falling back to `child-src`, then `default-src`, as browsers do), and an
+  explicit `frame-src` may not list an origin no `<iframe>` on the page uses. `base-uri`
+  must contain `'none'` or `'self'`, and `form-action` must
   be present. Each rule has a fixture self-test. Zero dependencies.
 - `node --test scripts/update-csp-hashes.test.mjs` — fixture self-test of
   `scripts/update-csp-hashes.mjs`, the tool that regenerates the `sha256-` allow-list the
diff --git a/scripts/page-csp.mjs b/scripts/page-csp.mjs
index bf4c972..c2f8418 100644
--- a/scripts/page-csp.mjs
+++ b/scripts/page-csp.mjs
@@ -48,6 +48,17 @@ export function externalImageOrigins(html) {
   return [...out].sort();
 }
 
+// Origins of every <iframe> whose src is absolute http(s); relative, srcdoc
+// and about: frames are same-document and governed by no fetch directive.
+export function externalFrameOrigins(html) {
+  const out = new Set();
+  for (const m of html.matchAll(/<iframe\b[^>]*>/gi)) {
+    const src = attrs(m[0]).src ?? "";
+    if (/^https?:\/\//i.test(src)) out.add(new URL(src).origin);
+  }
+  return [...out].sort();
+}
+
 // The first CSP <meta> in <head>: its full tag text, parsed directives (in
 // source order), and where it and the first fetch-triggering tag sit in <head>.
 export function parseCsp(html) {
@@ -118,6 +129,18 @@ export function problemsOf(html) {
   for (const origin of externalImageOrigins(html)) {
     if (!imgSrc.includes(origin) && !imgSrc.includes("https:")) problems.push(`img-src does not allow external <img> origin ${origin}`);
   }
+  // Browsers resolve frame-src -> child-src -> default-src (CSP3). A frame
+  // whose origin is not allowed is silently replaced by an empty box.
+  const frameSrc = d["frame-src"] ?? d["child-src"] ?? d["default-src"] ?? [];
+  const frameOrigins = externalFrameOrigins(html);
+  for (const origin of frameOrigins) {
+    if (!frameSrc.includes(origin) && !frameSrc.includes("https:")) problems.push(`frame-src does not allow external <iframe> origin ${origin}`);
+  }
+  // An explicit frame-src is the page's third-party-frame allow-list: an
+  // origin no <iframe> points at widens it for nothing (like a stale hash).
+  for (const origin of d["frame-src"] ?? []) {
+    if (/^https?:\/\//i.test(origin) && !frameOrigins.includes(origin)) problems.push(`frame-src allows ${origin} but no <iframe> on the page uses it`);
+  }
   if (!(d["base-uri"] ?? []).some((t) => t === "'none'" || t === "'self'")) problems.push("base-uri must be 'none' or 'self'");
   if (!d["form-action"]) problems.push("form-action directive missing");
   return problems;
diff --git a/scripts/page-csp.test.mjs b/scripts/page-csp.test.mjs
index 3364755..f2f776c 100644
--- a/scripts/page-csp.test.mjs
+++ b/scripts/page-csp.test.mjs
@@ -6,15 +6,16 @@
 // test recomputes every hash from the committed HTML and fails on mismatch
 // (scripts/update-csp-hashes.mjs rewrites the policy; the helpers live in
 // scripts/page-csp.mjs).
-// It also keeps the policy from regressing to 'unsafe-inline'/'unsafe-eval'
-// and keeps every external <img> origin covered by img-src.
+// It also keeps the policy from regressing to 'unsafe-inline'/'unsafe-eval',
+// keeps every external <img> origin covered by img-src and every external
+// <iframe> origin covered by frame-src (with no frame-src origin left unused).
 // Zero dependencies. Usage: node --test scripts/page-csp.test.mjs
 import { test } from "node:test";
 import assert from "node:assert/strict";
 import { readFileSync } from "node:fs";
 import { relative } from "node:path";
 import { ROOT, htmlFiles, isRedirectPage } from "./html-pages.mjs";
-import { REGEN_HINT, externalImageOrigins, inlineScriptSources, parseCsp, problemsOf, sha256Source } from "./page-csp.mjs";
+import { REGEN_HINT, externalFrameOrigins, externalImageOrigins, inlineScriptSources, parseCsp, problemsOf, sha256Source } from "./page-csp.mjs";
 
 // --- per-page gates ---------------------------------------------------------
 
@@ -32,9 +33,9 @@ for (const file of htmlFiles()) {
 
 // --- fixture self-tests -----------------------------------------------------
 
-function fixture({ csp, script = "console.log(1)", handler = "", img = "" } = {}) {
+function fixture({ csp, script = "console.log(1)", handler = "", img = "", frame = "" } = {}) {
   const meta = csp === null ? "" : `<meta http-equiv="Content-Security-Policy" content="${csp}">`;
-  return `<!doctype html><html><head><meta charset="utf-8">${meta}<title>x</title></head><body>${img}<p${handler ? ` onerror="${handler}"` : ""}></p><script type="application/ld+json">{"@context":"https://schema.org","@type":"Thing"}</script><script>${script}</script></body></html>`;
+  return `<!doctype html><html><head><meta charset="utf-8">${meta}<title>x</title></head><body>${img}${frame}<p${handler ? ` onerror="${handler}"` : ""}></p><script type="application/ld+json">{"@context":"https://schema.org","@type":"Thing"}</script><script>${script}</script></body></html>`;
 }
 
 const BASE = "default-src 'none'; img-src 'self'; base-uri 'none'; form-action 'none'";
@@ -186,6 +187,44 @@ test("fixture: external <img> origin must be in img-src", () => {
   assert.deepEqual(problemsOf(fixture({ csp: `default-src 'none'; img-src 'self' https://github.com; base-uri 'none'; form-action 'none'; script-src ${h}`, img })), []);
 });
 
+const FRAME = '<iframe src="https://www.youtube-nocookie.com/embed/x" title="x"></iframe>';
+
+test("fixture: <iframe> without src, relative src and srcdoc contribute no external origin", () => {
+  assert.deepEqual(externalFrameOrigins('<iframe title="a"></iframe><iframe src="/embed.html" title="b"></iframe><iframe srcdoc="<p>x</p>" title="c"></iframe><IFRAME SRC="HTTPS://Example.org/e?x=1" title="d"></IFRAME>'), ["https://example.org"]);
+  const h = sha256Source("console.log(1)");
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; script-src ${h}`, frame: '<iframe src="/embed.html" title="x"></iframe>' })), []);
+});
+
+test("fixture: external <iframe> origin must be in frame-src", () => {
+  const h = sha256Source("console.log(1)");
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; frame-src https://example.org; script-src ${h}`, frame: FRAME })), [
+    "frame-src does not allow external <iframe> origin https://www.youtube-nocookie.com",
+    "frame-src allows https://example.org but no <iframe> on the page uses it",
+  ]);
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; frame-src https://www.youtube-nocookie.com; script-src ${h}`, frame: FRAME })), []);
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; frame-src https:; script-src ${h}`, frame: FRAME })), []);
+});
+
+test("fixture: without frame-src, child-src then default-src governs <iframe> origins", () => {
+  const h = sha256Source("console.log(1)");
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; child-src https://www.youtube-nocookie.com; script-src ${h}`, frame: FRAME })), []);
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; script-src ${h}`, frame: FRAME })), [
+    "frame-src does not allow external <iframe> origin https://www.youtube-nocookie.com",
+  ]);
+  assert.deepEqual(problemsOf(fixture({ csp: `img-src 'self'; base-uri 'none'; form-action 'none'; script-src ${h}`, frame: FRAME })), [
+    "default-src must be 'none' so every fetch type is allow-listed explicitly",
+    "frame-src does not allow external <iframe> origin https://www.youtube-nocookie.com",
+  ]);
+});
+
+test("fixture: a frame-src origin no <iframe> uses is reported; 'self' and 'none' are not origins", () => {
+  const h = sha256Source("console.log(1)");
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; frame-src 'self' https://www.youtube-nocookie.com; script-src ${h}` })), [
+    "frame-src allows https://www.youtube-nocookie.com but no <iframe> on the page uses it",
+  ]);
+  assert.deepEqual(problemsOf(fixture({ csp: `${BASE}; frame-src 'none'; script-src ${h}` })), []);
+});
+
 test("fixture: policy placed after a <script> in <head> is reported", () => {
   const h = sha256Source("console.log(1)");
   const html = `<!doctype html><html><head><meta charset="utf-8"><script>console.log(1)</script><meta http-equiv="Content-Security-Policy" content="${BASE}; script-src ${h}"></head><body></body></html>`;