diff --git a/web/src/components/sections/Hero.test.tsx b/web/src/components/sections/Hero.test.tsx
new file mode 100644
index 000000000..5a2212e39
--- /dev/null
+++ b/web/src/components/sections/Hero.test.tsx
@@ -0,0 +1,103 @@
+import type { AnchorHTMLAttributes, ReactNode } from 'react';
+import { renderToString } from 'react-dom/server';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { act, render } from '@testing-library/react';
+import Hero from './Hero';
+
+// The hero's video is a YouTube embed that starts by itself and has no control
+// to stop it. Someone whose system asks for reduced motion gets the poster
+// under it and no embed at all, and the server, which cannot know, sends the
+// poster alone.
+
+vi.mock('next/link', () => ({
+  default: ({
+    href,
+    children,
+    ...rest
+  }: AnchorHTMLAttributes<HTMLAnchorElement> & { href: string; children: ReactNode }) => (
+    <a href={href} {...rest}>
+      {children}
+    </a>
+  ),
+}));
+
+vi.mock('@/lib/posthogEvents', () => ({ captureEvent: () => {} }));
+
+const REDUCE = '(prefers-reduced-motion: reduce)';
+
+// One reduced-motion setting for the page, which a test can flip the way the
+// system settings would, firing change on every query that listens.
+let reduce = false;
+const listeners = new Set<() => void>();
+function setReduce(next: boolean) {
+  reduce = next;
+  act(() => listeners.forEach((fn) => fn()));
+}
+
+beforeEach(() => {
+  vi.useFakeTimers({ toFake: ['setTimeout', 'clearTimeout'] });
+  reduce = false;
+  listeners.clear();
+  vi.stubGlobal('matchMedia', (query: string) => ({
+    media: query,
+    get matches() {
+      return query === REDUCE ? reduce : false;
+    },
+    addEventListener: (_: string, fn: () => void) => listeners.add(fn),
+    removeEventListener: (_: string, fn: () => void) => listeners.delete(fn),
+  }));
+});
+
+afterEach(() => {
+  vi.useRealTimers();
+  vi.unstubAllGlobals();
+});
+
+const poster = (root: ParentNode) => root.querySelector('img[src="/product_v2.jpg"]');
+const video = (root: ParentNode) => root.querySelector('iframe');
+
+describe('Hero', () => {
+  it('plays the video over the poster, hidden while the player loads', () => {
+    const { container } = render(<Hero />);
+    expect(poster(container)).not.toBeNull();
+    expect(video(container)?.getAttribute('src')).toContain('youtube.com/embed/');
+    expect(video(container)?.style.opacity).toBe('0');
+
+    act(() => vi.advanceTimersByTime(3000));
+    expect(video(container)?.style.opacity).toBe('1');
+
+    act(() => vi.advanceTimersByTime(89000));
+    expect(video(container)?.style.opacity).toBe('0');
+  });
+
+  it('shows the poster alone when reduced motion is on', () => {
+    reduce = true;
+    const { container } = render(<Hero />);
+    expect(poster(container)).not.toBeNull();
+    expect(video(container)).toBeNull();
+
+    act(() => vi.advanceTimersByTime(10000));
+    expect(video(container)).toBeNull();
+  });
+
+  it('drops the video when reduced motion turns on, and starts it hidden again when it turns off', () => {
+    const { container } = render(<Hero />);
+    act(() => vi.advanceTimersByTime(3000));
+    expect(video(container)?.style.opacity).toBe('1');
+
+    setReduce(true);
+    expect(video(container)).toBeNull();
+    expect(poster(container)).not.toBeNull();
+
+    setReduce(false);
+    expect(video(container)?.style.opacity).toBe('0');
+    act(() => vi.advanceTimersByTime(3000));
+    expect(video(container)?.style.opacity).toBe('1');
+  });
+
+  it('sends the poster without the embed from the server', () => {
+    const html = renderToString(<Hero />);
+    expect(html).toContain('/product_v2.jpg');
+    expect(html).not.toContain('<iframe');
+  });
+});
diff --git a/web/src/components/sections/Hero.tsx b/web/src/components/sections/Hero.tsx
index 3dae1cf9a..f54d60ecc 100644
--- a/web/src/components/sections/Hero.tsx
+++ b/web/src/components/sections/Hero.tsx
@@ -1,6 +1,6 @@
 'use client';
 
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useSyncExternalStore } from 'react';
 import HeroJournalLink from "@/components/journal/HeroJournalLink";
 import { captureEvent } from "@/lib/posthogEvents";
 import { CROWD_SUPPLY_URL } from "@/lib/crowdSupply";
@@ -10,7 +10,22 @@ import { CROWD_SUPPLY_URL } from "@/lib/crowdSupply";
 // said as "around", because it is an estimate.
 const PEOPLE_WHO_MADE_ONE = 40;
 
-export default function Hero() {
+// The embed starts by itself and has no control to stop it, so someone who has
+// asked their system for less motion gets the poster and no embed at all. The
+// server cannot know, so it sends the poster alone and the client adds the
+// video after hydration.
+const REDUCED_MOTION = '(prefers-reduced-motion: reduce)';
+function subscribeReducedMotion(onChange: () => void) {
+  const mq = window.matchMedia(REDUCED_MOTION);
+  mq.addEventListener('change', onChange);
+  return () => mq.removeEventListener('change', onChange);
+}
+const getReducedMotion = () => window.matchMedia(REDUCED_MOTION).matches;
+const getReducedMotionServer = () => true;
+
+// Mounted only while motion is welcome. Turning reduced motion off again
+// mounts a fresh one, which hides for its first three seconds like the first.
+function HeroVideo() {
   const [isVideoVisible, setIsVideoVisible] = useState(false);
 
   useEffect(() => {
@@ -30,6 +45,31 @@ export default function Hero() {
     };
   }, []);
 
+  return (
+    <iframe
+      src="https://www.youtube.com/embed/OXt-yg_7qdk?autoplay=1&mute=1&controls=0&modestbranding=1&disablekb=1&playsinline=1&rel=0"
+      title="Patternflow Demo Video"
+      frameBorder="0"
+      allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
+      allowFullScreen
+      style={{
+        position: 'absolute',
+        top: 0,
+        left: 0,
+        width: '100%',
+        height: '100%',
+        display: 'block',
+        pointerEvents: 'none',
+        opacity: isVideoVisible ? 1 : 0,
+        transition: 'opacity 0.6s ease-in-out',
+      }}
+    />
+  );
+}
+
+export default function Hero() {
+  const reducedMotion = useSyncExternalStore(subscribeReducedMotion, getReducedMotion, getReducedMotionServer);
+
   return (
     <section className="hero">
       <HeroJournalLink />
@@ -66,24 +106,7 @@ export default function Hero() {
               display: 'block' 
             }} 
           />
-          <iframe 
-            src="https://www.youtube.com/embed/OXt-yg_7qdk?autoplay=1&mute=1&controls=0&modestbranding=1&disablekb=1&playsinline=1&rel=0" 
-            title="Patternflow Demo Video"
-            frameBorder="0" 
-            allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" 
-            allowFullScreen
-            style={{
-              position: 'absolute',
-              top: 0,
-              left: 0,
-              width: '100%',
-              height: '100%',
-              display: 'block',
-              pointerEvents: 'none',
-              opacity: isVideoVisible ? 1 : 0,
-              transition: 'opacity 0.6s ease-in-out',
-            }}
-          />
+          {!reducedMotion && <HeroVideo />}
         </div>
         {/* L3 — the strongest line we have, per the manifesto: a description of
             how the system works, with nothing in it to disbelieve. */}
diff --git a/web/src/components/sections/RightPanel.test.tsx b/web/src/components/sections/RightPanel.test.tsx
new file mode 100644
index 000000000..902f4195b
--- /dev/null
+++ b/web/src/components/sections/RightPanel.test.tsx
@@ -0,0 +1,93 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { fireEvent, render, screen, within } from '@testing-library/react';
+import type { SectionContent } from '@/lib/content';
+import RightPanel from './RightPanel';
+
+// The section tabs on /, /build, /pattern and /inside. The desktop column and
+// the phone's bar are two navs with one label, and CSS shows one at a time;
+// either way the open tab reads as pressed, its ✕ stays out of its name, and
+// the panel under them is the page's main landmark.
+
+vi.mock('./Hero', () => ({ default: () => <section>hero</section> }));
+vi.mock('./Milestones', () => ({ default: () => null }));
+vi.mock('./Sponsor', () => ({ default: () => null }));
+vi.mock('../layout/Footer', () => ({ default: () => null }));
+vi.mock('./BuildPanel', () => ({ default: () => <section>build panel</section> }));
+vi.mock('./PatternPanel', () => ({ default: () => <section>pattern panel</section> }));
+vi.mock('./InsidePanel', () => ({ default: () => <section>inside panel</section> }));
+vi.mock('@/lib/posthogEvents', () => ({ captureEvent: () => {} }));
+
+const content: SectionContent = { title: '', subtitle: '', content: '' };
+
+beforeEach(() => {
+  vi.stubGlobal('matchMedia', (query: string) => ({
+    media: query,
+    matches: false,
+    addEventListener: () => {},
+    removeEventListener: () => {},
+  }));
+  vi.stubGlobal('scrollTo', () => {});
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+function renderAt(path: string, tab: 'hero' | 'build' | 'pattern' | 'inside') {
+  window.history.replaceState(null, '', path);
+  return render(
+    <RightPanel initialTab={tab} buildContent={content} patternContent={content} insideContent={content} />,
+  );
+}
+
+const pressed = (nav: HTMLElement) =>
+  within(nav)
+    .getAllByRole('button')
+    .filter((b) => b.getAttribute('aria-pressed') === 'true')
+    .map((b) => b.textContent?.replace('✕', ''));
+
+describe('RightPanel', () => {
+  it('names both tab rows Sections and marks the open tab pressed in each', () => {
+    renderAt('/build', 'build');
+    const navs = screen.getAllByRole('navigation', { name: 'Sections' });
+    expect(navs).toHaveLength(2);
+    for (const nav of navs) {
+      expect(pressed(nav)).toEqual(['Build']);
+      for (const name of ['Pattern', 'Inside']) {
+        expect(within(nav).getByRole('button', { name })).toHaveAttribute('aria-pressed', 'false');
+      }
+    }
+  });
+
+  it('keeps the ✕ on the open desktop tab out of its name', () => {
+    renderAt('/build', 'build');
+    const [desktop] = screen.getAllByRole('navigation', { name: 'Sections' });
+    const build = within(desktop).getByRole('button', { name: 'Build' });
+    expect(build).toHaveTextContent('✕');
+    expect(build).toHaveAttribute('aria-pressed', 'true');
+  });
+
+  it('moves pressed with a click, and a second click on the open tab closes it', () => {
+    renderAt('/', 'hero');
+    const [desktop, phone] = screen.getAllByRole('navigation', { name: 'Sections' });
+    expect(pressed(desktop)).toEqual([]);
+
+    fireEvent.click(within(desktop).getByRole('button', { name: 'Pattern' }));
+    expect(pressed(desktop)).toEqual(['Pattern']);
+    expect(pressed(phone)).toEqual(['Pattern']);
+    expect(window.location.pathname).toBe('/pattern');
+
+    fireEvent.click(within(desktop).getByRole('button', { name: 'Pattern' }));
+    expect(pressed(desktop)).toEqual([]);
+    expect(window.location.pathname).toBe('/');
+  });
+
+  it('puts the panels in one main, outside the tabs', () => {
+    renderAt('/inside', 'inside');
+    const main = screen.getByRole('main');
+    expect(main).toHaveTextContent('inside panel');
+    for (const nav of screen.getAllByRole('navigation', { name: 'Sections' })) {
+      expect(main.contains(nav)).toBe(false);
+    }
+  });
+});
diff --git a/web/src/components/sections/RightPanel.tsx b/web/src/components/sections/RightPanel.tsx
index 3a0743e7a..7a3f57dd3 100644
--- a/web/src/components/sections/RightPanel.tsx
+++ b/web/src/components/sections/RightPanel.tsx
@@ -45,7 +45,7 @@ function tabFromPath(pathname: string): TabType {
 export default function RightPanel({ initialTab = 'hero', buildContent, patternContent, insideContent }: RightPanelProps) {
   const [activeTab, setActiveTab] = useState<TabType>(initialTab);
   const [buildPanelKey, setBuildPanelKey] = useState(0);
-  const contentRef = useRef<HTMLDivElement>(null);
+  const contentRef = useRef<HTMLElement>(null);
   const activeTabRef = useRef<TabType>(initialTab);
 
   // Apply a tab change to local + global state (no URL update).
@@ -130,32 +130,38 @@ export default function RightPanel({ initialTab = 'hero', buildContent, patternC
 
   return (
     <div className="right-panel-layout">
-      <div className="vertical-nav">
+      {/* The desktop column. It carries the phone bar's label: CSS shows one
+          of the two, so a screen reader meets one Sections nav either way. The
+          open tab is pressed, as on the phone, and its ✕ is only drawn. */}
+      <nav className="vertical-nav" aria-label="Sections">
         <button 
           className={`v-tab-btn ${activeTab === 'build' ? 'active' : ''}`}
+          aria-pressed={activeTab === 'build'}
           onClick={() => handleTabClick('build')}
         >
-          {activeTab === 'build' && <span className="close-icon">✕</span>}
+          {activeTab === 'build' && <span className="close-icon" aria-hidden="true">✕</span>}
           <span className="tab-text" data-mobile-label="1. Build">Build</span>
         </button>
 
         <button 
           className={`v-tab-btn ${activeTab === 'pattern' ? 'active' : ''}`}
+          aria-pressed={activeTab === 'pattern'}
           onClick={() => handleTabClick('pattern')}
         >
-          {activeTab === 'pattern' && <span className="close-icon">✕</span>}
+          {activeTab === 'pattern' && <span className="close-icon" aria-hidden="true">✕</span>}
           <span className="tab-text" data-mobile-label="2. Pattern">Pattern</span>
         </button>
         <button 
           className={`v-tab-btn ${activeTab === 'inside' ? 'active' : ''}`}
+          aria-pressed={activeTab === 'inside'}
           onClick={() => handleTabClick('inside')}
         >
-          {activeTab === 'inside' && <span className="close-icon">✕</span>}
+          {activeTab === 'inside' && <span className="close-icon" aria-hidden="true">✕</span>}
           <span className="tab-text" data-mobile-label="3. Inside">Inside</span>
         </button>
-      </div>
+      </nav>
 
-      <div className={`content-panel ${activeTab !== 'hero' ? 'bg-white viewer-open' : ''}`} ref={contentRef}>
+      <main className={`content-panel ${activeTab !== 'hero' ? 'bg-white viewer-open' : ''}`} ref={contentRef}>
         <div className="deck-content">
           {/* The panels not on screen are only faded and folded away, so they
               are inert too: out of the tab order and the accessibility tree.
@@ -177,7 +183,7 @@ export default function RightPanel({ initialTab = 'hero', buildContent, patternC
             <InsidePanel content={insideContent} />
           </div>
         </div>
-      </div>
+      </main>
 
       {/* Mobile-only bottom nav. Tapping a section drops the 3D viewer down from
           the top; tapping the active one again returns to the full-screen hero. */}