diff --git a/src/browser/tests/document/create_element.html b/src/browser/tests/document/create_element.html
index b8f17815..00f84b70 100644
--- a/src/browser/tests/document/create_element.html
+++ b/src/browser/tests/document/create_element.html
@@ -2,12 +2,16 @@
 <body></body>
 <script src="../testing.js"></script>
 <script id=createElement>
-  const div = document.createElement('div');
-  testing.expectEqual("DIV", div.tagName);
-  div.id = "hello";
+  const div1 = document.createElement('div');
+  testing.expectEqual(true, div1 instanceof HTMLDivElement);
+  testing.expectEqual("DIV", div1.tagName);
+  div1.id = "hello";
   testing.expectEqual(null, $('#hello'));
 
-  document.getElementsByTagName('body')[0].appendChild(div);
-  testing.expectEqual(div, $('#hello'));
+  const div2 = document.createElement('DIV');
+  testing.expectEqual(true, div2 instanceof HTMLDivElement);
+
+  document.getElementsByTagName('body')[0].appendChild(div1);
+  testing.expectEqual(div1, $('#hello'));
 </script>
 
diff --git a/src/browser/tests/document/create_element_ns.html b/src/browser/tests/document/create_element_ns.html
index 123ccaac..73b3ee24 100644
--- a/src/browser/tests/document/create_element_ns.html
+++ b/src/browser/tests/document/create_element_ns.html
@@ -2,9 +2,15 @@
 <body></body>
 <script src="../testing.js"></script>
 <script id=createElementNS>
-  const htmlDiv = document.createElementNS('http://www.w3.org/1999/xhtml', 'div');
-  testing.expectEqual('DIV', htmlDiv.tagName);
-  testing.expectEqual('http://www.w3.org/1999/xhtml', htmlDiv.namespaceURI);
+  const htmlDiv1 = document.createElementNS('http://www.w3.org/1999/xhtml', 'div');
+  testing.expectEqual('DIV', htmlDiv1.tagName);
+  testing.expectEqual(true, htmlDiv1 instanceof HTMLDivElement);
+  testing.expectEqual('http://www.w3.org/1999/xhtml', htmlDiv1.namespaceURI);
+
+  const htmlDiv2 = document.createElementNS('http://www.w3.org/1999/xhtml', 'DIV');
+  testing.expectEqual('DIV', htmlDiv2.tagName);
+  testing.expectEqual(true, htmlDiv2 instanceof HTMLDivElement);
+  testing.expectEqual('http://www.w3.org/1999/xhtml', htmlDiv2.namespaceURI);
 
   const svgRect = document.createElementNS('http://www.w3.org/2000/svg', 'RecT');
   testing.expectEqual('RecT', svgRect.tagName);
diff --git a/src/browser/webapi/Document.zig b/src/browser/webapi/Document.zig
index f428b1f1..33181cea 100644
--- a/src/browser/webapi/Document.zig
+++ b/src/browser/webapi/Document.zig
@@ -125,14 +125,16 @@ const CreateElementOptions = struct {
 
 pub fn createElement(self: *Document, name: []const u8, options_: ?CreateElementOptions, page: *Page) !*Element {
     try validateElementName(name);
-    const namespace: Element.Namespace = blk: {
+    const ns: Element.Namespace, const normalized_name = blk: {
         if (self._type == .html) {
-            break :blk .html;
+            break :blk .{ .html, std.ascii.lowerString(&page.buf, name) };
         }
         // Generic and XML documents create XML elements
-        break :blk .xml;
+        break :blk .{ .xml, name };
     };
-    const node = try page.createElementNS(namespace, name, null);
+    // HTML documents are case-insensitive - lowercase the tag name
+
+    const node = try page.createElementNS(ns, normalized_name, null);
     const element = node.as(Element);
 
     // Track owner document if it's not the main document
@@ -151,7 +153,9 @@ pub fn createElement(self: *Document, name: []const u8, options_: ?CreateElement
 
 pub fn createElementNS(self: *Document, namespace: ?[]const u8, name: []const u8, page: *Page) !*Element {
     try validateElementName(name);
-    const node = try page.createElementNS(Element.Namespace.parse(namespace), name, null);
+    const ns = Element.Namespace.parse(namespace);
+    const normalized_name = if (ns == .html) std.ascii.lowerString(&page.buf, name) else name;
+    const node = try page.createElementNS(ns, normalized_name, null);
 
     // Track owner document if it's not the main document
     if (self != page.document) {
