check submit element and form into Form.requestSubmit

src/browser/tests/element/html/form.html

@@ -384,3 +384,82 @@
   testing.expectEqual(true, true);
 }
 </script>
+
+<!-- Test: requestSubmit() with non-submit-button submitter throws TypeError -->
+<form id="test_form_rs1" action="/should-not-navigate4" method="get">
+  <input id="rs1_text" type="text" name="q" value="test">
+  <input id="rs1_submit" type="submit" value="Go">
+  <input id="rs1_image" type="image" src="x.png">
+  <button id="rs1_btn_submit" type="submit">Submit</button>
+  <button id="rs1_btn_reset" type="reset">Reset</button>
+  <button id="rs1_btn_button" type="button">Button</button>
+</form>
+
+<script id="requestSubmit_rejects_non_submit_button">
+{
+  const form = $('#test_form_rs1');
+  form.addEventListener('submit', (e) => e.preventDefault());
+
+  // A text input is not a submit button — should throw TypeError
+  testing.expectError('TypeError', () => {
+    form.requestSubmit($('#rs1_text'));
+  });
+
+  // A reset button is not a submit button — should throw TypeError
+  testing.expectError('TypeError', () => {
+    form.requestSubmit($('#rs1_btn_reset'));
+  });
+
+  // A <button type="button"> is not a submit button — should throw TypeError
+  testing.expectError('TypeError', () => {
+    form.requestSubmit($('#rs1_btn_button'));
+  });
+
+  // A <div> is not a submit button — should throw TypeError
+  const div = document.createElement('div');
+  form.appendChild(div);
+  testing.expectError('TypeError', () => {
+    form.requestSubmit(div);
+  });
+}
+</script>
+
+<!-- Test: requestSubmit() accepts valid submit buttons -->
+<script id="requestSubmit_accepts_submit_buttons">
+{
+  const form = $('#test_form_rs1');
+  let submitCount = 0;
+  form.addEventListener('submit', (e) => { e.preventDefault(); submitCount++; });
+
+  // <input type="submit"> is a valid submitter
+  form.requestSubmit($('#rs1_submit'));
+  testing.expectEqual(1, submitCount);
+
+  // <input type="image"> is a valid submitter
+  form.requestSubmit($('#rs1_image'));
+  testing.expectEqual(2, submitCount);
+
+  // <button type="submit"> is a valid submitter
+  form.requestSubmit($('#rs1_btn_submit'));
+  testing.expectEqual(3, submitCount);
+}
+</script>
+
+<!-- Test: requestSubmit() with submitter not owned by form throws NotFoundError -->
+<form id="test_form_rs2" action="/should-not-navigate5" method="get">
+  <input type="text" name="q" value="test">
+</form>
+<form id="test_form_rs3">
+  <input id="rs3_submit" type="submit" value="Other Submit">
+</form>
+
+<script id="requestSubmit_rejects_wrong_form_submitter">
+{
+  const form = $('#test_form_rs2');
+
+  // Submit button belongs to a different form — should throw NotFoundError
+  testing.expectError('NotFoundError', () => {
+    form.requestSubmit($('#rs3_submit'));
+  });
+}
+</script>

src/browser/webapi/element/html/Form.zig

@@ -120,11 +120,44 @@ pub fn submit(self: *Form, page: *Page) !void {
 /// https://html.spec.whatwg.org/multipage/forms.html#dom-form-requestsubmit
 /// Like submit(), but fires the submit event and validates the form.
 pub fn requestSubmit(self: *Form, submitter: ?*Element, page: *Page) !void {
-    // TODO check the submitter is a submit button if not null.
-    const submitter_element = submitter orelse self.asElement();
+    const submitter_element = if (submitter) |s| blk: {
+        // The submitter must be a submit button.
+        if (!isSubmitButton(s)) return error.TypeError;
+
+        // The submitter's form owner must be this form element.
+        const submitter_form = getFormOwner(s, page);
+        if (submitter_form == null or submitter_form.? != self) return error.NotFound;
+
+        break :blk s;
+    } else self.asElement();
+
     return page.submitForm(submitter_element, self, .{});
 }
 
+/// Returns true if the element is a submit button per the HTML spec:
+/// - <input type="submit"> or <input type="image">
+/// - <button type="submit"> (including default, since button's default type is "submit")
+fn isSubmitButton(element: *Element) bool {
+    if (element.is(Input)) |input| {
+        return input._input_type == .submit or input._input_type == .image;
+    }
+    if (element.is(Button)) |button| {
+        return std.mem.eql(u8, button.getType(), "submit");
+    }
+    return false;
+}
+
+/// Returns the form owner of a submittable element (Input or Button).
+fn getFormOwner(element: *Element, page: *Page) ?*Form {
+    if (element.is(Input)) |input| {
+        return input.getForm(page);
+    }
+    if (element.is(Button)) |button| {
+        return button.getForm(page);
+    }
+    return null;
+}
+
 pub const JsApi = struct {
     pub const bridge = js.Bridge(Form);
     pub const Meta = struct {
@@ -140,7 +173,7 @@ pub const JsApi = struct {
     pub const elements = bridge.accessor(Form.getElements, null, .{});
     pub const length = bridge.accessor(Form.getLength, null, .{});
     pub const submit = bridge.function(Form.submit, .{});
-    pub const requestSubmit = bridge.function(Form.requestSubmit, .{});
+    pub const requestSubmit = bridge.function(Form.requestSubmit, .{ .dom_exception = true });
 };
 
 const testing = @import("../../../../testing.zig");