diff --git a/packages/dev-test/src/env/browser.spec.ts b/packages/dev-test/src/env/browser.spec.ts
index 30fa22b..423448e 100644
--- a/packages/dev-test/src/env/browser.spec.ts
+++ b/packages/dev-test/src/env/browser.spec.ts
@@ -24,6 +24,18 @@ describe('browser', () => {
     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('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 0a702a0..e51de80 100644
--- a/packages/dev-test/src/env/browser.ts
+++ b/packages/dev-test/src/env/browser.ts
@@ -22,8 +22,12 @@ export function browser () {
   const { window } = new JSDOM('', { url: 'http://localhost' });
 
   return {
-    // Event constructors, see BROWSER_OVERRIDES
+    // The DOM node hierarchy, which libraries check with instanceof, and the
+    // event constructors (Node has its own of these, see BROWSER_OVERRIDES)
     CustomEvent: window.CustomEvent,
+    Document: window.Document,
+    DocumentFragment: window.DocumentFragment,
+    Element: window.Element,
     Event: window.Event,
     // All HTML Elements that are defined on the JSDOM window object.
     // (we copied as-is from the types definition). We cannot get this
@@ -99,6 +103,11 @@ export function browser () {
     HTMLUListElement: window.HTMLUListElement,
     HTMLUnknownElement: window.HTMLUnknownElement,
     HTMLVideoElement: window.HTMLVideoElement,
+    // the rest of the node hierarchy
+    Node: window.Node,
+    SVGElement: window.SVGElement,
+    ShadowRoot: window.ShadowRoot,
+    Text: window.Text,
     // normal service resumes, the base top-level names
     crypto: window.crypto,
     document: window.document,
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),