diff --git a/src/browser/tests/css/stylesheet.html b/src/browser/tests/css/stylesheet.html
index b9176351..8e284e66 100644
--- a/src/browser/tests/css/stylesheet.html
+++ b/src/browser/tests/css/stylesheet.html
@@ -447,6 +447,27 @@
 }
 </script>
 
+<script id="CSSStyleSheet_insertRule_default_index">
+{
+    const style = document.createElement('style');
+    document.head.appendChild(style);
+    const sheet = style.sheet;
+
+    testing.expectEqual(0, sheet.cssRules.length);
+
+    // Call without index, should default to 0
+    sheet.insertRule('.test-default { color: blue; }');
+    testing.expectEqual(1, sheet.cssRules.length);
+    testing.expectEqual('.test-default', sheet.cssRules[0].selectorText);
+
+    // Insert another rule without index, should default to 0 and push the first one to index 1
+    sheet.insertRule('.test-at-0 { color: red; }');
+    testing.expectEqual(2, sheet.cssRules.length);
+    testing.expectEqual('.test-at-0', sheet.cssRules[0].selectorText);
+    testing.expectEqual('.test-default', sheet.cssRules[1].selectorText);
+}
+</script>
+
 <script id="CSSStyleSheet_replaceSync">
 {
     const sheet = new CSSStyleSheet();
diff --git a/src/browser/webapi/css/CSSStyleSheet.zig b/src/browser/webapi/css/CSSStyleSheet.zig
index 70f9c2ad..33f3e609 100644
--- a/src/browser/webapi/css/CSSStyleSheet.zig
+++ b/src/browser/webapi/css/CSSStyleSheet.zig
@@ -55,7 +55,8 @@ pub fn getOwnerRule(self: *const CSSStyleSheet) ?*CSSRule {
     return self._owner_rule;
 }
 
-pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, index: u32, page: *Page) !u32 {
+pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, maybe_index: ?u32, page: *Page) !u32 {
+    const index = maybe_index orelse 0;
     var it = Parser.parseStylesheet(rule);
     const parsed_rule = it.next() orelse return error.SyntaxError;