diff --git a/package.json b/package.json
index bb88130..d0caa8f 100644
--- a/package.json
+++ b/package.json
@@ -30,7 +30,7 @@
     "docs": "pezkuwi-dev-build-docs",
     "lint": "pezkuwi-dev-run-lint",
     "postinstall": "./packages/dev/scripts/pezkuwi-dev-yarn-only.mjs",
-    "test": "yarn build && pezkuwi-dev-run-test --dev-build --env browser"
+    "test": "yarn build && pezkuwi-dev-run-test --dev-build --experimental-test-module-mocks --env browser"
   },
   "devDependencies": {
     "@pezkuwi/dev": "workspace:packages/dev",
diff --git a/packages/dev-test/package.json b/packages/dev-test/package.json
index 03d6f04..ed29e2a 100644
--- a/packages/dev-test/package.json
+++ b/packages/dev-test/package.json
@@ -15,7 +15,7 @@
   },
   "sideEffects": false,
   "type": "module",
-  "version": "0.86.5",
+  "version": "0.86.6",
   "main": "./index.js",
   "exports": {
     "./globals.d.ts": "./src/globals.d.ts"
diff --git a/packages/dev-test/src/env/browser.spec.ts b/packages/dev-test/src/env/browser.spec.ts
index e38c910..47f0018 100644
--- a/packages/dev-test/src/env/browser.spec.ts
+++ b/packages/dev-test/src/env/browser.spec.ts
@@ -23,4 +23,42 @@ describe('browser', () => {
   it('contains HTML*Element', () => {
     expect(typeof all.HTMLElement).toBe('function');
   });
+
+  it('contains the DOM node hierarchy', () => {
+    const div = document.createElement('div');
+
+    expect(div).toBeInstanceOf(Element);
+    expect(div).toBeInstanceOf(Node);
+    expect(document.createTextNode('text')).toBeInstanceOf(Text);
+    expect(document.createDocumentFragment()).toBeInstanceOf(DocumentFragment);
+    expect(document.createElementNS('http://www.w3.org/2000/svg', 'svg')).toBeInstanceOf(SVGElement);
+    expect(div.attachShadow({ mode: 'open' })).toBeInstanceOf(ShadowRoot);
+    expect(document).toBeInstanceOf(Document);
+  });
+
+  it('contains the UI event constructors', () => {
+    const div = document.createElement('div');
+
+    expect(() => div.dispatchEvent(new MouseEvent('click'))).not.toThrow();
+    expect(() => div.dispatchEvent(new KeyboardEvent('keydown', { key: 'Enter' }))).not.toThrow();
+    expect(new FocusEvent('focus')).toBeInstanceOf(UIEvent);
+  });
+
+  it('contains getComputedStyle and the observers', () => {
+    const div = document.createElement('div');
+
+    div.style.display = 'none';
+    document.body.appendChild(div);
+
+    expect(getComputedStyle(div).display).toBe('none');
+    expect(() => new MutationObserver(() => undefined).observe(div, { childList: true })).not.toThrow();
+    expect(() => new ResizeObserver(() => undefined).observe(div)).not.toThrow();
+
+    div.remove();
+  });
+
+  it('dispatches the global CustomEvent and Event on window', () => {
+    expect(() => window.dispatchEvent(new CustomEvent('custom', { detail: 1 }))).not.toThrow();
+    expect(() => window.dispatchEvent(new Event('plain'))).not.toThrow();
+  });
 });
diff --git a/packages/dev-test/src/env/browser.ts b/packages/dev-test/src/env/browser.ts
index 39377a1..fda9a8b 100644
--- a/packages/dev-test/src/env/browser.ts
+++ b/packages/dev-test/src/env/browser.ts
@@ -1,8 +1,80 @@
 // Copyright 2017-2026 @pezkuwi/dev-test authors & contributors
 // SPDX-License-Identifier: Apache-2.0
 
+import type { DOMWindow } from 'jsdom';
+
 import { JSDOM } from 'jsdom';
 
+/**
+ * Constructors that Node defines as well. A DOM node only accepts an event
+ * created in its own realm, so in the browser environment the JSDOM ones
+ * replace Node's, as they do under jest-environment-jsdom.
+ */
+export const BROWSER_OVERRIDES: readonly string[] = ['CustomEvent', 'Event'];
+
+/**
+ * JSDOM does no layout, so no element ever changes size and the callback
+ * never runs. JSDOM has no ResizeObserver of its own.
+ */
+class ResizeObserver {
+  observe (): void {
+    // nothing is ever resized
+  }
+
+  unobserve (): void {
+    // nothing is being watched
+  }
+
+  disconnect (): void {
+    // nothing is being watched
+  }
+}
+
+/** The DOM node hierarchy, which libraries check with instanceof */
+function nodes (window: DOMWindow) {
+  return {
+    CharacterData: window.CharacterData,
+    Comment: window.Comment,
+    Document: window.Document,
+    DocumentFragment: window.DocumentFragment,
+    Element: window.Element,
+    Node: window.Node,
+    NodeList: window.NodeList,
+    SVGElement: window.SVGElement,
+    ShadowRoot: window.ShadowRoot,
+    Text: window.Text
+  };
+}
+
+/** Event constructors. Node has its own Event and CustomEvent, see BROWSER_OVERRIDES */
+function events (window: DOMWindow) {
+  return {
+    CompositionEvent: window.CompositionEvent,
+    CustomEvent: window.CustomEvent,
+    Event: window.Event,
+    FocusEvent: window.FocusEvent,
+    InputEvent: window.InputEvent,
+    KeyboardEvent: window.KeyboardEvent,
+    MouseEvent: window.MouseEvent,
+    TouchEvent: window.TouchEvent,
+    UIEvent: window.UIEvent,
+    WheelEvent: window.WheelEvent
+  };
+}
+
+/** Styles, geometry and the observers */
+function layout (window: DOMWindow) {
+  return {
+    // present on the JSDOM window, absent from its DOMWindow type
+    DOMRect: window['DOMRect'] as typeof DOMRect,
+    DOMRectReadOnly: window['DOMRectReadOnly'] as typeof DOMRectReadOnly,
+    MutationObserver: window.MutationObserver,
+    MutationRecord: window.MutationRecord,
+    ResizeObserver,
+    getComputedStyle: window.getComputedStyle.bind(window)
+  };
+}
+
 /**
  * Export a very basic JSDom environment - this is just enough so we have
  * @testing-environment/react tests passing in this repo
@@ -15,6 +87,9 @@ export function browser () {
   const { window } = new JSDOM('', { url: 'http://localhost' });
 
   return {
+    ...nodes(window),
+    ...events(window),
+    ...layout(window),
     // All HTML Elements that are defined on the JSDOM window object.
     // (we copied as-is from the types definition). We cannot get this
     // via Object.keys(window).filter(...) so we have to specify explicitly
diff --git a/packages/dev-test/src/env/expect.spec.ts b/packages/dev-test/src/env/expect.spec.ts
index 2de1cf8..b14a0c3 100644
--- a/packages/dev-test/src/env/expect.spec.ts
+++ b/packages/dev-test/src/env/expect.spec.ts
@@ -255,4 +255,28 @@ describe('expect', () => {
       expect(() => expect([1, 2]).not.toEqual(expect.arrayContaining([2]))).toThrow();
     });
   });
+
+  describe('.toContain', () => {
+    it('finds a substring in a string', () => {
+      expect('voter-alice').toContain('alice');
+      expect(() => expect('voter-alice').toContain('bob')).toThrow();
+    });
+
+    it('finds the item itself in an array or other iterable', () => {
+      const item = { a: 1 };
+
+      expect([1, item]).toContain(item);
+      expect(new Set(['a'])).toContain('a');
+      expect(() => expect([{ a: 1 }]).toContain({ a: 1 })).toThrow();
+    });
+
+    it('rejects a value that is neither a string nor an iterable', () => {
+      expect(() => expect(123).toContain(1)).toThrow();
+    });
+
+    it('supports .not', () => {
+      expect('voter-alice').not.toContain('bob');
+      expect(() => expect([1, 2]).not.toContain(2)).toThrow();
+    });
+  });
 });
diff --git a/packages/dev-test/src/env/expect.ts b/packages/dev-test/src/env/expect.ts
index 72ef0d0..2a2fec9 100644
--- a/packages/dev-test/src/env/expect.ts
+++ b/packages/dev-test/src/env/expect.ts
@@ -144,6 +144,40 @@ function assertMatchObj (value: unknown, check: object): void {
     .forEach(([key, other]) => assertMatch((value as Record<string, unknown>)[key], other));
 }
 
+/**
+ * @internal
+ *
+ * Whether a string contains the substring, or another iterable the item
+ */
+function contains (value: unknown, item: unknown): boolean {
+  if (typeof value === 'string') {
+    return typeof item === 'string' && value.includes(item);
+  }
+
+  assert.ok(typeof (value as Iterable<unknown> | null | undefined)?.[Symbol.iterator] === 'function', `Expected a string or an iterable, found ${typeof value}`);
+
+  return [...(value as Iterable<unknown>)].includes(item);
+}
+
+/**
+ * @internal
+ *
+ * A helper for toContain: a string contains the substring, any other
+ * iterable contains the item itself (not a deep-equal copy)
+ */
+function assertContains (value: unknown, item: unknown): void {
+  assert.ok(contains(value, item), `Expected ${String(value)} to contain ${String(item)}`);
+}
+
+/**
+ * @internal
+ *
+ * The inverse of assertContains
+ */
+function assertNotContains (value: unknown, item: unknown): void {
+  assert.ok(!contains(value, item), `Expected ${String(value)} not to contain ${String(item)}`);
+}
+
 /**
  * @internal
  *
@@ -269,6 +303,7 @@ export function expect () {
           toBeDefined: () => assert.ok(value === undefined),
           toBeNull: (value: unknown) => assert.ok(value !== null),
           toBeUndefined: () => assert.ok(value !== undefined),
+          toContain: (item: unknown) => assertNotContains(value, item),
           toEqual: (other: unknown) => containsMatcher(other)
             ? assert.throws(() => assertEqualMatch(value, other))
             : assert.notDeepEqual(value, other),
@@ -287,6 +322,7 @@ export function expect () {
         toBeNull: (value: unknown) => assert.ok(value === null),
         toBeTruthy: () => assert.ok(value),
         toBeUndefined: () => assert.ok(value === undefined),
+        toContain: (item: unknown) => assertContains(value, item),
         toEqual: (other: unknown) => containsMatcher(other)
           ? assertEqualMatch(value, other)
           : assert.deepEqual(value, other),
diff --git a/packages/dev-test/src/env/index.ts b/packages/dev-test/src/env/index.ts
index 37ce94e..8ec9132 100644
--- a/packages/dev-test/src/env/index.ts
+++ b/packages/dev-test/src/env/index.ts
@@ -1,7 +1,7 @@
 // Copyright 2017-2026 @pezkuwi/dev-test authors & contributors
 // SPDX-License-Identifier: Apache-2.0
 
-import { browser } from './browser.js';
+import { browser, BROWSER_OVERRIDES } from './browser.js';
 import { expect } from './expect.js';
 import { jest } from './jest.js';
 import { lifecycle } from './lifecycle.js';
@@ -15,7 +15,11 @@ export function exposeEnv (isBrowser: boolean): void {
     env && Object
       .entries(env())
       .forEach(([key, fn]) => {
-        globalThis[key as 'undefined'] ??= fn;
+        if (isBrowser && BROWSER_OVERRIDES.includes(key)) {
+          (globalThis as Record<string, unknown>)[key] = fn;
+        } else {
+          globalThis[key as 'undefined'] ??= fn;
+        }
       });
   });
 }
diff --git a/packages/dev-test/src/env/moduleMocks.spec.ts b/packages/dev-test/src/env/moduleMocks.spec.ts
new file mode 100644
index 0000000..c4fb1b6
--- /dev/null
+++ b/packages/dev-test/src/env/moduleMocks.spec.ts
@@ -0,0 +1,14 @@
+// Copyright 2017-2026 @pezkuwi/dev-test authors & contributors
+// SPDX-License-Identifier: Apache-2.0
+
+import { mock } from 'node:test';
+
+describe('mock.module', () => {
+  it('replaces a module loaded after the mock is registered', async () => {
+    mock.module('node:os', { namedExports: { platform: () => 'mocked' } });
+
+    const { platform } = await import('node:os');
+
+    expect(platform()).toBe('mocked');
+  });
+});
diff --git a/packages/dev-test/src/packageInfo.ts b/packages/dev-test/src/packageInfo.ts
index 058817c..65cf17e 100644
--- a/packages/dev-test/src/packageInfo.ts
+++ b/packages/dev-test/src/packageInfo.ts
@@ -3,4 +3,4 @@
 
 // Do not edit, auto-generated by @pezkuwi/dev
 
-export const packageInfo = { name: '@pezkuwi/dev-test', path: 'auto', type: 'auto', version: '0.86.5' };
+export const packageInfo = { name: '@pezkuwi/dev-test', path: 'auto', type: 'auto', version: '0.86.6' };
diff --git a/packages/dev-ts/package.json b/packages/dev-ts/package.json
index 6a93946..d600536 100644
--- a/packages/dev-ts/package.json
+++ b/packages/dev-ts/package.json
@@ -15,7 +15,7 @@
   },
   "sideEffects": false,
   "type": "module",
-  "version": "0.86.5",
+  "version": "0.86.6",
   "main": "./index.js",
   "exports": {
     "./globals.d.ts": "./src/globals.d.ts"
diff --git a/packages/dev-ts/src/packageInfo.ts b/packages/dev-ts/src/packageInfo.ts
index 58403f8..e5aec23 100644
--- a/packages/dev-ts/src/packageInfo.ts
+++ b/packages/dev-ts/src/packageInfo.ts
@@ -3,4 +3,4 @@
 
 // Do not edit, auto-generated by @pezkuwi/dev
 
-export const packageInfo = { name: '@pezkuwi/dev-ts', path: 'auto', type: 'auto', version: '0.86.5' };
+export const packageInfo = { name: '@pezkuwi/dev-ts', path: 'auto', type: 'auto', version: '0.86.6' };
diff --git a/packages/dev/README.md b/packages/dev/README.md
index 4038350..c0d1d7d 100644
--- a/packages/dev/README.md
+++ b/packages/dev/README.md
@@ -252,6 +252,11 @@ This script runs test files in the repository, filtering by file extensions and
 - **`--logfile <file>`**:  
   Specifies a log file to capture test output.
 
+- **`--experimental-test-module-mocks`**:  
+  Passed to Node.js, enabling `mock.module` from `node:test`. Node only swaps a module that has not
+  been loaded yet, so a spec registers its mocks first and then loads the code under test with a
+  dynamic `import()`.
+
 - **`--import <module>`**:  
   Imports the specified module.
 
diff --git a/packages/dev/package.json b/packages/dev/package.json
index 6cc6e20..559a971 100644
--- a/packages/dev/package.json
+++ b/packages/dev/package.json
@@ -15,7 +15,7 @@
   },
   "sideEffects": false,
   "type": "module",
-  "version": "0.86.5",
+  "version": "0.86.6",
   "bin": {
     "pezkuwi-ci-ghact-build": "./scripts/pezkuwi-ci-ghact-build.mjs",
     "pezkuwi-ci-ghact-docs": "./scripts/pezkuwi-ci-ghact-docs.mjs",
@@ -52,8 +52,8 @@
   "dependencies": {
     "@eslint/compat": "^2.1.1",
     "@eslint/js": "^10.0.1",
-    "@pezkuwi/dev-test": "^0.86.5",
-    "@pezkuwi/dev-ts": "^0.86.5",
+    "@pezkuwi/dev-test": "^0.86.6",
+    "@pezkuwi/dev-ts": "^0.86.6",
     "@rollup/plugin-alias": "^5.1.1",
     "@rollup/plugin-commonjs": "^29.0.3",
     "@rollup/plugin-dynamic-import-vars": "^2.1.5",
diff --git a/packages/dev/scripts/pezkuwi-dev-run-test.mjs b/packages/dev/scripts/pezkuwi-dev-run-test.mjs
index 0e2878a..9e45fa5 100755
--- a/packages/dev/scripts/pezkuwi-dev-run-test.mjs
+++ b/packages/dev/scripts/pezkuwi-dev-run-test.mjs
@@ -58,6 +58,11 @@ for (let i = 0; i < args.length; i++) {
       cmd.push(args[++i]);
       break;
 
+    // node flags with no params
+    case '--experimental-test-module-mocks':
+      nodeFlags.push(args[i]);
+      break;
+
     // node flags that could have additional params
     case '--import':
     case '--loader':
diff --git a/packages/dev/src/packageInfo.ts b/packages/dev/src/packageInfo.ts
index 8f31535..1ec83e0 100644
--- a/packages/dev/src/packageInfo.ts
+++ b/packages/dev/src/packageInfo.ts
@@ -3,4 +3,4 @@
 
 // Do not edit, auto-generated by @pezkuwi/dev
 
-export const packageInfo = { name: '@pezkuwi/dev', path: 'auto', type: 'auto', version: '0.86.5' };
+export const packageInfo = { name: '@pezkuwi/dev', path: 'auto', type: 'auto', version: '0.86.6' };
diff --git a/yarn.lock b/yarn.lock
index 9a80126..ace19c3 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1189,7 +1189,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@pezkuwi/dev-test@npm:^0.86.5, @pezkuwi/dev-test@workspace:packages/dev-test":
+"@pezkuwi/dev-test@npm:^0.86.6, @pezkuwi/dev-test@workspace:packages/dev-test":
   version: 0.0.0-use.local
   resolution: "@pezkuwi/dev-test@workspace:packages/dev-test"
   dependencies:
@@ -1205,7 +1205,7 @@ __metadata:
   languageName: unknown
   linkType: soft
 
-"@pezkuwi/dev-ts@npm:^0.86.5, @pezkuwi/dev-ts@workspace:packages/dev-ts":
+"@pezkuwi/dev-ts@npm:^0.86.6, @pezkuwi/dev-ts@workspace:packages/dev-ts":
   version: 0.0.0-use.local
   resolution: "@pezkuwi/dev-ts@workspace:packages/dev-ts"
   dependencies:
@@ -1221,8 +1221,8 @@ __metadata:
   dependencies:
     "@eslint/compat": "npm:^2.1.1"
     "@eslint/js": "npm:^10.0.1"
-    "@pezkuwi/dev-test": "npm:^0.86.5"
-    "@pezkuwi/dev-ts": "npm:^0.86.5"
+    "@pezkuwi/dev-test": "npm:^0.86.6"
+    "@pezkuwi/dev-ts": "npm:^0.86.6"
     "@rollup/plugin-alias": "npm:^5.1.1"
     "@rollup/plugin-commonjs": "npm:^29.0.3"
     "@rollup/plugin-dynamic-import-vars": "npm:^2.1.5"