diff --git a/benchmark/fetch/headers.js b/benchmark/fetch/headers.js
new file mode 100644
index 000000000000..4ff5091ebfbf
--- /dev/null
+++ b/benchmark/fetch/headers.js
@@ -0,0 +1,94 @@
+'use strict';
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  n: [1e5],
+  method: [
+    'construct-empty',
+    'construct-object',
+    'construct-headers',
+    'get',
+    'get-common',
+    'set',
+    'append',
+    'has',
+    'delete',
+    'iterate',
+  ],
+});
+
+const objectInit = {
+  'Accept': 'application/json',
+  'Content-Type': 'text/plain',
+  'User-Agent': 'benchmark',
+  'Authorization': 'Bearer token',
+  'Cookie': 'a=1',
+  'X-Request-Id': 'abc',
+  'Cache-Control': 'no-cache',
+  'Host': 'example.com',
+};
+
+function main({ n, method }) {
+  const headers = new Headers(objectInit);
+  const copySource = new Headers(objectInit);
+  let result;
+
+  bench.start();
+  switch (method) {
+    case 'construct-empty':
+      for (let i = 0; i < n; i++)
+        new Headers();
+      break;
+    case 'construct-object':
+      for (let i = 0; i < n; i++)
+        new Headers(objectInit);
+      break;
+    case 'construct-headers':
+      for (let i = 0; i < n; i++)
+        new Headers(copySource);
+      break;
+    case 'get':
+      for (let i = 0; i < n; i++)
+        result = headers.get('x-request-id');
+      break;
+    case 'get-common':
+      for (let i = 0; i < n; i++)
+        result = headers.get('content-type');
+      break;
+    case 'set':
+      for (let i = 0; i < n; i++)
+        headers.set('x-count', i);
+      break;
+    case 'append':
+      for (let i = 0; i < n; i++) {
+        const current = new Headers();
+        current.append('Accept', 'text/html');
+        current.append('X-Custom', i);
+      }
+      break;
+    case 'has':
+      for (let i = 0; i < n; i++)
+        result = headers.has('authorization');
+      break;
+    case 'delete': {
+      for (let i = 0; i < n; i++) {
+        const current = new Headers(objectInit);
+        current.delete('content-type');
+      }
+      break;
+    }
+    case 'iterate':
+      for (let i = 0; i < n; i++) {
+        for (const entry of headers)
+          result = entry;
+      }
+      break;
+    default:
+      throw new Error(`Unexpected method "${method}"`);
+  }
+  bench.end(n);
+
+  // Keep a live use so V8 cannot DCE the loop.
+  if (result === Symbol.for('benchmark-never'))
+    throw new Error('unreachable');
+}
diff --git a/test/benchmark/test-benchmark-fetch.js b/test/benchmark/test-benchmark-fetch.js
new file mode 100644
index 000000000000..e9c686003a01
--- /dev/null
+++ b/test/benchmark/test-benchmark-fetch.js
@@ -0,0 +1,7 @@
+'use strict';
+
+require('../common');
+
+const runBenchmark = require('../common/benchmark');
+
+runBenchmark('fetch', { NODEJS_BENCHMARK_ZERO_ALLOWED: 1 });
diff --git a/test/parallel/test-whatwg-headers.js b/test/parallel/test-whatwg-headers.js
new file mode 100644
index 000000000000..94610128ce11
--- /dev/null
+++ b/test/parallel/test-whatwg-headers.js
@@ -0,0 +1,245 @@
+'use strict';
+
+// Tests below are not from WPT.
+
+require('../common');
+const assert = require('assert');
+const util = require('util');
+
+{
+  const headers = new Headers();
+  assert.strictEqual(headers.get('content-type'), null);
+  assert.strictEqual(headers.has('content-type'), false);
+  assert.deepStrictEqual([...headers], []);
+  assert.deepStrictEqual(headers.getSetCookie(), []);
+}
+
+{
+  const headers = new Headers({
+    'Content-Type': 'text/plain',
+    'Accept': 'application/json',
+    'X-Custom': '1',
+  });
+  assert.strictEqual(headers.get('content-type'), 'text/plain');
+  assert.strictEqual(headers.get('Content-Type'), 'text/plain');
+  assert.strictEqual(headers.get('ACCEPT'), 'application/json');
+  assert.ok(headers.has('accept'));
+  assert.deepStrictEqual([...headers], [
+    ['accept', 'application/json'],
+    ['content-type', 'text/plain'],
+    ['x-custom', '1'],
+  ]);
+}
+
+{
+  const headers = new Headers([
+    ['X-A', '1'],
+    ['x-b', '2'],
+    ['X-A', '3'],
+  ]);
+  assert.strictEqual(headers.get('x-a'), '1, 3');
+  assert.deepStrictEqual([...headers], [
+    ['x-a', '1, 3'],
+    ['x-b', '2'],
+  ]);
+}
+
+{
+  const source = new Headers({ 'Content-Type': 'text/html' });
+  source.append('Set-Cookie', 'a=b');
+  source.append('Set-Cookie', 'c=d');
+  const copy = new Headers(source);
+  assert.strictEqual(copy.get('content-type'), 'text/html');
+  assert.deepStrictEqual(copy.getSetCookie(), ['a=b', 'c=d']);
+  assert.deepStrictEqual([...copy], [...source]);
+  copy.append('X-Copy', 'yes');
+  assert.strictEqual(source.has('x-copy'), false);
+  source.append('Set-Cookie', 'e=f');
+  assert.deepStrictEqual(copy.getSetCookie(), ['a=b', 'c=d']);
+}
+
+{
+  const headers = new Headers();
+  headers.append('Accept', 'text/html');
+  headers.append('accept', 'application/json');
+  assert.strictEqual(headers.get('ACCEPT'), 'text/html, application/json');
+  headers.set('ACCEPT', 'image/png');
+  assert.strictEqual(headers.get('accept'), 'image/png');
+  headers.delete('Accept');
+  assert.strictEqual(headers.has('accept'), false);
+}
+
+{
+  const headers = new Headers();
+  headers.append('Cookie', 'a=1');
+  headers.append('cookie', 'b=2');
+  assert.strictEqual(headers.get('cookie'), 'a=1; b=2');
+}
+
+{
+  const headers = new Headers();
+  headers.append('set-cookie', 'a=b');
+  headers.append('Set-Cookie', 'c=d');
+  assert.deepStrictEqual(headers.getSetCookie(), ['a=b', 'c=d']);
+  const cloned = headers.getSetCookie();
+  cloned.push('e=f');
+  assert.deepStrictEqual(headers.getSetCookie(), ['a=b', 'c=d']);
+  headers.set('set-cookie', 'only=one');
+  assert.deepStrictEqual(headers.getSetCookie(), ['only=one']);
+  headers.delete('SET-COOKIE');
+  assert.deepStrictEqual(headers.getSetCookie(), []);
+}
+
+{
+  const headers = new Headers();
+  headers.set('a', '  value  ');
+  assert.strictEqual(headers.get('a'), 'value');
+  headers.set('b', '\r\n\t  trimmed\t\n');
+  assert.strictEqual(headers.get('b'), 'trimmed');
+  headers.set('c', '\r');
+  assert.strictEqual(headers.get('c'), '');
+  headers.set('d', '\n');
+  assert.strictEqual(headers.get('d'), '');
+}
+
+{
+  const headers = new Headers();
+  headers.set('a', ['b', 'c']);
+  assert.strictEqual(headers.get('a'), 'b,c');
+  headers.set('b', null);
+  assert.strictEqual(headers.get('b'), 'null');
+  headers.set('c', 1);
+  assert.strictEqual(headers.get('c'), '1');
+}
+
+{
+  const headers = new Headers({
+    c: '5',
+    b: ['3', '4'],
+    a: ['1', '2'],
+  });
+  assert.deepStrictEqual([...headers.entries()], [
+    ['a', '1,2'],
+    ['b', '3,4'],
+    ['c', '5'],
+  ]);
+}
+
+{
+  const init = [
+    ['foo', '123'],
+    ['bar', '456'],
+  ];
+  const headers = new Headers(init);
+  for (const [key, val] of headers) {
+    headers.delete(key);
+    headers.set(`x-${key}`, val);
+  }
+  assert.deepStrictEqual([...headers], [
+    ['foo', '123'],
+    ['x-x-bar', '456'],
+  ]);
+}
+
+{
+  const headers = new Headers([
+    ['b', '2'],
+    ['c', '3'],
+    ['e', '5'],
+  ]);
+  headers.append('d', '4');
+  headers.append('a', '1');
+  headers.append('f', '6');
+  headers.append('c', '7');
+  headers.append('abc', '8');
+  assert.deepStrictEqual([...headers], [
+    ['a', '1'],
+    ['abc', '8'],
+    ['b', '2'],
+    ['c', '3, 7'],
+    ['d', '4'],
+    ['e', '5'],
+    ['f', '6'],
+  ]);
+}
+
+{
+  const headers = new Headers({ 'Content-Type': 'application/json' });
+  headers.set('Authorization', 'Bearer token');
+  assert.strictEqual(
+    util.inspect(headers, { depth: 1 }),
+    "Headers { 'Content-Type': 'application/json', Authorization: 'Bearer token' }",
+  );
+}
+
+{
+  const headers = new Headers();
+  assert.throws(() => headers.get(), TypeError);
+  assert.throws(() => headers.has(), TypeError);
+  assert.throws(() => headers.delete(), TypeError);
+  assert.throws(() => headers.append('a'), TypeError);
+  assert.throws(() => headers.set('a'), TypeError);
+  assert.throws(() => headers.append('invalid @ name', 'x'), TypeError);
+  assert.throws(() => headers.set('a', 'a\nb'), TypeError);
+  assert.throws(() => headers.set('a', 'a\rb'), TypeError);
+  assert.throws(() => headers.set('a', 'a\0b'), TypeError);
+  assert.throws(() => headers.set(Symbol('x'), 'y'), TypeError);
+  assert.throws(() => headers.set('a', Symbol('y')), TypeError);
+  assert.throws(() => headers.set('', 'x'), TypeError);
+  assert.throws(() => headers.set('a', 'héllo\u0100'), TypeError);
+  assert.throws(() => new Headers(1), TypeError);
+  assert.throws(() => new Headers('1'), TypeError);
+  assert.throws(() => new Headers([['undici', 'fetch'], ['fetch']]), TypeError);
+}
+
+{
+  assert.throws(() => Headers.prototype.get.call(null, 'a'), {
+    name: 'TypeError',
+    code: 'ERR_INVALID_THIS',
+  });
+  assert.throws(() => Headers.prototype.append.call({}, 'a', 'b'), {
+    name: 'TypeError',
+    code: 'ERR_INVALID_THIS',
+  });
+}
+
+{
+  assert.strictEqual(Headers.prototype.append.length, 2);
+  assert.strictEqual(Headers.prototype.constructor.length, 0);
+  assert.strictEqual(Headers.prototype.delete.length, 1);
+  assert.strictEqual(Headers.prototype.get.length, 1);
+  assert.strictEqual(Headers.prototype.has.length, 1);
+  assert.strictEqual(Headers.prototype.set.length, 2);
+  assert.strictEqual(Headers.prototype.entries, Headers.prototype[Symbol.iterator]);
+  assert.strictEqual(Headers.prototype[Symbol.toStringTag], 'Headers');
+  assert.strictEqual(Object.prototype.toString.call(Headers.prototype), '[object Headers]');
+}
+
+{
+  const headers = new Headers();
+  headers.set('content-type', 'text/plain');
+  assert.strictEqual(headers.delete('content-type'), undefined);
+  assert.strictEqual(headers.delete('missing'), undefined);
+  assert.strictEqual(headers.set('a', 'b'), undefined);
+}
+
+{
+  const headers = new Headers();
+  for (const name of [
+    'content-type',
+    'accept',
+    'user-agent',
+    'cache-control',
+    'set-cookie',
+  ]) {
+    headers.set(name, 'value');
+    assert.strictEqual(headers.get(name), 'value');
+    assert.ok(headers.has(name));
+  }
+}
+
+{
+  const headers = new Headers();
+  headers.append('fhqwhgads', `a${'\t'.repeat(1000)}a`);
+  assert.strictEqual(headers.get('fhqwhgads'), `a${'\t'.repeat(1000)}a`);
+}