diff --git a/packages/postcss-normalize-url/src/normalize.js b/packages/postcss-normalize-url/src/normalize.js
index 41031843a..e6597e0de 100644
--- a/packages/postcss-normalize-url/src/normalize.js
+++ b/packages/postcss-normalize-url/src/normalize.js
@@ -1,3 +1,4 @@
+'use strict';
 /* Derived from normalize-url https://github.com/sindresorhus/normalize-url/main/index.js by Sindre Sorhus */
 
 // https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs
diff --git a/packages/postcss-normalize-url/test/normalize.js b/packages/postcss-normalize-url/test/normalize.js
new file mode 100644
index 000000000..5fe6b742c
--- /dev/null
+++ b/packages/postcss-normalize-url/test/normalize.js
@@ -0,0 +1,73 @@
+'use strict';
+
+const assert = require('uvu/assert');
+const { test } = require('uvu');
+const normalizeUrl = require('../src/normalize.js');
+
+test('should add the http prefix to unprefixed URLs', () => {
+  assert.is(normalizeUrl('example.com'), 'http://example.com');
+});
+
+test('should not attempt to sort parameters', () => {
+  const fixture = 'http://sindresorhus.com/?d=Z&b=Y&c=X&a=W';
+  assert.is(normalizeUrl(fixture), fixture);
+});
+
+test('should leave encoded slashes alone', () => {
+  const fixture = 'https://example.com/music/bands/AC%2FDC';
+  assert.is(normalizeUrl(fixture), fixture);
+});
+
+test('should decode URI octets', () => {
+  assert.is(
+    normalizeUrl('http://example.com/%7Efoo/'),
+    'http://example.com/~foo'
+  );
+});
+
+test('should handle spaces inside parameters', () => {
+  assert.is(
+    normalizeUrl('http://example.com/?foo=bar baz'),
+    'http://example.com/?foo=bar%20baz'
+  );
+});
+
+test('should preserve authentication string', () => {
+  const fixture = 'http://user:password@www.example.com';
+  assert.is(normalizeUrl(fixture), fixture);
+});
+
+test('should preserve index', () => {
+  const fixture = 'http://example.com/index.html';
+  assert.is(normalizeUrl(fixture), fixture);
+});
+
+test('should preserve non-standard port', () => {
+  const fixture = 'https://example.com:123';
+  assert.is(normalizeUrl(fixture), fixture);
+});
+
+test('should strip default MIME type', () => {
+  assert.is(normalizeUrl('data:text/plain,foo'), 'data:,foo');
+});
+
+test('should strip default charset', () => {
+  assert.is(normalizeUrl('data:;charset=us-ascii,foo'), 'data:,foo');
+});
+
+test('should lowercase the MIME type', () => {
+  assert.is(normalizeUrl('data:TEXT/HTML,foo'), 'data:text/html,foo');
+});
+
+test('should keep spaces when not base64', () => {
+  assert.is(normalizeUrl('data:, foo #bar'), 'data:, foo #bar');
+});
+
+test('should remove trailing semicolon', () => {
+  assert.is(
+    normalizeUrl('data:;charset=UTF-8;,foo'),
+    'data:;charset=utf-8,foo'
+  );
+});
+
+test.run();