diff --git a/tests/manifest-contract.test.mjs b/tests/manifest-contract.test.mjs
new file mode 100644
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+++ b/tests/manifest-contract.test.mjs
@@ -0,0 +1,224 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { readdirSync, readFileSync, statSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import { join, posix } from 'node:path';
+
+const repoRoot = fileURLToPath(new URL('..', import.meta.url));
+const staticRoot = join(repoRoot, 'static');
+const manifestPath = join(staticRoot, 'manifest.json');
+
+const manifestSource = readFileSync(manifestPath, 'utf8');
+
+// https://www.w3.org/TR/appmanifest/#display-member
+const DISPLAY_MODES = new Set([
+  'fullscreen',
+  'standalone',
+  'minimal-ui',
+  'browser',
+]);
+
+const EXTENSION_FOR_TYPE = {
+  'image/svg+xml': '.svg',
+  'image/png': '.png',
+  'image/webp': '.webp',
+  'image/x-icon': '.ico',
+};
+
+const HEX_COLOR = /^#[0-9a-fA-F]{6}$/;
+
+// Per the appmanifest spec, member URLs resolve against the manifest's own
+// URL, so both "/favicons/x.svg" and "favicons/x.svg" are valid and both land
+// under static/ here (the manifest sits at static/manifest.json). Only a
+// scheme-bearing URL leaves the site, which is what this rejects.
+function hasScheme(value) {
+  return /^[a-zA-Z][a-zA-Z0-9+.-]*:/.test(value) || value.startsWith('//');
+}
+
+// Docusaurus copies static/ into build/ verbatim, and GitHub Pages serves from
+// a case-sensitive filesystem. existsSync() on a case-insensitive development
+// machine happily resolves /favicons/Favicon.svg, so each segment is compared
+// against the real directory entry instead.
+function resolveCaseExact(reference) {
+  const segments = reference.replace(/^\//, '').split('/').filter(Boolean);
+  let current = staticRoot;
+  for (const segment of segments) {
+    const entries = readdirSync(current);
+    assert.ok(
+      entries.includes(segment),
+      `${reference}: "${segment}" is not present in static/${posix.relative(
+        staticRoot,
+        current,
+      )} case-exactly. Directory holds: ${entries.join(', ')}`,
+    );
+    current = join(current, segment);
+  }
+  return current;
+}
+
+function pngDimensions(buffer) {
+  assert.equal(
+    buffer.subarray(1, 4).toString('latin1'),
+    'PNG',
+    'expected a PNG signature',
+  );
+  return { width: buffer.readUInt32BE(16), height: buffer.readUInt32BE(20) };
+}
+
+// Parsed lazily and defensively: a syntax error must surface as a failure of
+// the named test below rather than as a module-load crash that reports every
+// other assertion in this file as "not run".
+let parseError = null;
+let manifest = {};
+try {
+  manifest = JSON.parse(manifestSource);
+} catch (error) {
+  parseError = error;
+}
+
+test('manifest.json parses as a JSON object', () => {
+  assert.equal(
+    parseError,
+    null,
+    `static/manifest.json must be valid JSON; a browser that cannot parse it drops the whole manifest silently: ${parseError?.message}`,
+  );
+  assert.equal(typeof manifest, 'object');
+  assert.ok(manifest !== null && !Array.isArray(manifest));
+});
+
+test('manifest declares the fields browsers need to offer installation', () => {
+  for (const field of ['name', 'short_name', 'start_url', 'display']) {
+    assert.equal(typeof manifest[field], 'string', `${field} must be a string`);
+    assert.ok(manifest[field].length > 0, `${field} must not be empty`);
+  }
+  assert.ok(
+    DISPLAY_MODES.has(manifest.display),
+    `display must be one of ${[...DISPLAY_MODES].join(', ')}, got ${manifest.display}`,
+  );
+  assert.ok(
+    !hasScheme(manifest.start_url),
+    'start_url must stay same-origin; an absolute URL points the installed app off-site',
+  );
+});
+
+test('manifest declares icons, so the assertions below cannot be vacuous', () => {
+  assert.ok(Array.isArray(manifest.icons), 'icons must be an array');
+  assert.ok(
+    manifest.icons.length > 0,
+    'icons must be non-empty; an empty array makes the site non-installable',
+  );
+});
+
+test('every icon entry is fully described', () => {
+  for (const icon of manifest.icons) {
+    assert.equal(typeof icon.src, 'string', 'icon.src must be a string');
+    assert.ok(
+      !hasScheme(icon.src),
+      `${icon.src} must be a same-origin path, not an absolute URL`,
+    );
+    assert.ok(
+      !icon.src.split('/').includes('..'),
+      `${icon.src} must not escape static/`,
+    );
+    assert.equal(
+      typeof icon.sizes,
+      'string',
+      `${icon.src}: sizes must be a string`,
+    );
+    assert.equal(
+      typeof icon.type,
+      'string',
+      `${icon.src}: type must be a string`,
+    );
+    assert.ok(
+      Object.hasOwn(EXTENSION_FOR_TYPE, icon.type),
+      `${icon.src}: unrecognised type ${icon.type}`,
+    );
+  }
+});
+
+test('every icon resolves under static/ case-exactly and is non-empty', () => {
+  for (const icon of manifest.icons) {
+    const resolved = resolveCaseExact(icon.src);
+    const stats = statSync(resolved);
+    assert.ok(stats.isFile(), `${icon.src} must resolve to a file`);
+    assert.ok(
+      stats.size > 0,
+      `${icon.src} is zero bytes; browsers treat it as a missing icon`,
+    );
+  }
+});
+
+test('each icon file matches the media type it is declared with', () => {
+  for (const icon of manifest.icons) {
+    const resolved = resolveCaseExact(icon.src);
+    assert.ok(
+      resolved.endsWith(EXTENSION_FOR_TYPE[icon.type]),
+      `${icon.src} is declared ${icon.type} but does not end in ${EXTENSION_FOR_TYPE[icon.type]}`,
+    );
+  }
+});
+
+test('raster icon dimensions match their declared sizes', () => {
+  for (const icon of manifest.icons.filter((i) => i.type === 'image/png')) {
+    assert.match(
+      icon.sizes,
+      /^\d+x\d+$/,
+      `${icon.src}: a raster icon needs concrete pixel sizes, got ${icon.sizes}`,
+    );
+    const [width, height] = icon.sizes.split('x').map(Number);
+    const actual = pngDimensions(readFileSync(resolveCaseExact(icon.src)));
+    assert.deepEqual(
+      actual,
+      { width, height },
+      `${icon.src} is ${actual.width}x${actual.height} on disk but declared ${icon.sizes}`,
+    );
+  }
+});
+
+test('vector icons declare sizes "any" rather than fixed pixels', () => {
+  for (const icon of manifest.icons.filter((i) => i.type === 'image/svg+xml')) {
+    assert.equal(
+      icon.sizes,
+      'any',
+      `${icon.src} is scalable, so pinning it to ${icon.sizes} hides it from every other size bucket`,
+    );
+  }
+});
+
+// Chromium requires a >=192px and a >=512px icon before it will fire
+// beforeinstallprompt. Losing either one removes the install affordance with
+// no build-time or runtime error.
+test('manifest carries icons large enough for an install prompt', () => {
+  const pixelSizes = manifest.icons
+    .filter((icon) => /^\d+x\d+$/.test(icon.sizes))
+    .map((icon) => Number(icon.sizes.split('x')[0]));
+  assert.ok(
+    pixelSizes.some((size) => size >= 192),
+    `needs an icon of at least 192px, have: ${pixelSizes.join(', ') || 'none'}`,
+  );
+  assert.ok(
+    pixelSizes.some((size) => size >= 512),
+    `needs an icon of at least 512px, have: ${pixelSizes.join(', ') || 'none'}`,
+  );
+});
+
+test('theme and background colors are six-digit hex', () => {
+  for (const field of ['background_color', 'theme_color']) {
+    if (manifest[field] === undefined) continue;
+    assert.match(
+      manifest[field],
+      HEX_COLOR,
+      `${field} must be a #rrggbb value; shorthand and named colors are unevenly supported`,
+    );
+  }
+});
+
+test('no two icons claim the same src', () => {
+  const sources = manifest.icons.map((icon) => icon.src);
+  assert.deepEqual(
+    sources,
+    [...new Set(sources)],
+    'duplicate icon src entries mean one of them is dead weight',
+  );
+});