diff --git a/Libraries/LibWeb/HTML/DOMStringList.cpp b/Libraries/LibWeb/HTML/DOMStringList.cpp
index 036f5174eb6..4e69f6b1fe4 100644
--- a/Libraries/LibWeb/HTML/DOMStringList.cpp
+++ b/Libraries/LibWeb/HTML/DOMStringList.cpp
@@ -43,7 +43,7 @@ Optional<String> DOMStringList::item(u32 index) const
 {
     // The item(index) method steps are to return the indexth item in this's associated list, or null if index plus one
     // is greater than this's associated list's size.
-    if (index + 1 > m_list.size())
+    if (index >= m_list.size())
         return {};
 
     return m_list.at(index);
@@ -58,7 +58,7 @@ bool DOMStringList::contains(StringView string)
 
 Optional<JS::Value> DOMStringList::item_value(size_t index) const
 {
-    if (index + 1 > m_list.size())
+    if (index >= m_list.size())
         return {};
 
     return JS::PrimitiveString::create(vm(), m_list.at(index));
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.txt b/Tests/LibWeb/Text/expected/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.txt
new file mode 100644
index 00000000000..ecce788eebf
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.txt
@@ -0,0 +1,9 @@
+Harness status: OK
+
+Found 4 tests
+
+4 Pass
+Pass	DOMStringList: length attribute
+Pass	DOMStringList: item() method
+Pass	DOMStringList: indexed getter
+Pass	DOMStringList: contains() method
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.html b/Tests/LibWeb/Text/input/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.html
new file mode 100644
index 00000000000..b426a13c331
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/infrastructure/common-dom-interfaces/collections/domstringlist.html
@@ -0,0 +1,61 @@
+<!doctype html>
+<title>DOMStringList</title>
+<script src="../../../../resources/testharness.js"></script>
+<script src="../../../../resources/testharnessreport.js"></script>
+<script>
+
+// Returns a promise that resolves to a DOMStringList with
+// the requested entries. Relies on Indexed DB.
+function createDOMStringList(entries) {
+  return new Promise((resolve, reject) => {
+    const dbname = String(self.location + Math.random());
+    const request = indexedDB.open(dbname);
+    request.onerror = () => reject(request.error);
+    request.onupgradeneeded = () => {
+      const db = request.result;
+      entries.forEach(entry => db.createObjectStore(entry));
+      const dsl = db.objectStoreNames;
+      resolve(dsl);
+      request.transaction.abort();
+    }
+  });
+}
+
+function dsl_test(entries, func, description) {
+  promise_test(t => createDOMStringList(entries).then(dsl => func(t, dsl)),
+               description);
+}
+
+dsl_test(['a', 'b', 'c'], (t, dsl) => {
+  assert_equals(dsl.length, 3, 'length attribute');
+}, 'DOMStringList: length attribute');
+
+dsl_test(['a', 'b', 'c'], (t, dsl) => {
+  assert_equals(dsl.item(0), 'a', 'item method');
+  assert_equals(dsl.item(1), 'b', 'item method');
+  assert_equals(dsl.item(2), 'c', 'item method');
+  assert_equals(dsl.item(3), null, 'item method out of range');
+  assert_equals(dsl.item(-1), null, 'item method out of range');
+  assert_throws_js(TypeError, () => dsl.item(),
+                   'item method should throw if called without enough args');
+}, 'DOMStringList: item() method');
+
+dsl_test(['a', 'b', 'c'], (t, dsl) => {
+  assert_equals(dsl[0], 'a', 'indexed getter');
+  assert_equals(dsl[1], 'b', 'indexed getter');
+  assert_equals(dsl[2], 'c', 'indexed getter');
+  assert_equals(dsl[3], undefined, 'indexed getter out of range');
+  assert_equals(dsl[-1], undefined, 'indexed getter out of range');
+}, 'DOMStringList: indexed getter');
+
+dsl_test(['a', 'b', 'c'], (t, dsl) => {
+  assert_true(dsl.contains('a'), 'contains method matched');
+  assert_true(dsl.contains('b'), 'contains method matched');
+  assert_true(dsl.contains('c'), 'contains method matched');
+  assert_false(dsl.contains(''), 'contains method unmatched');
+  assert_false(dsl.contains('d'), 'contains method unmatched');
+  assert_throws_js(TypeError, () => dsl.contains(),
+    'contains method should throw if called without enough args');
+}, 'DOMStringList: contains() method');
+
+</script>
