LibWeb: Implement HTMLInputElement's selected coordinates

When an <input type=image> button is clicked, we now send the (x,y)
coordinates of the click event (relative to the image) along with the
form submission data.

Regarding the text test, we can currently only test this feature with
dialogs. The headless-browser test infrastructure cannot yet handle the
resulting navigation that would occur if we were to test with normal
form submission.

Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp

@@ -189,12 +189,22 @@ WebIDL::ExceptionOr<void> HTMLFormElement::submit_form(JS::NonnullGCPtr<HTMLElem
         // 3. Let result be null.
         Optional<String> result;
 
-        // FIXME: 4. If submitter is an input element whose type attribute is in the Image Button state, then:
-        //           1. Let (x, y) be the selected coordinate.
-        //           2. Set result to the concatenation of x, ",", and y.
+        // 4. If submitter is an input element whose type attribute is in the Image Button state, then:
+        if (is<HTMLInputElement>(*submitter)) {
+            auto const& input_element = static_cast<HTMLInputElement const&>(*submitter);
+
+            if (input_element.type_state() == HTMLInputElement::TypeAttributeState::ImageButton) {
+                // 1. Let (x, y) be the selected coordinate.
+                auto [x, y] = input_element.selected_coordinate();
+
+                // 2. Set result to the concatenation of x, ",", and y.
+                result = MUST(String::formatted("{},{}", x, y));
+            }
+        }
 
         // 5. Otherwise, if submitter has a value, then set result to that value.
-        result = submitter->get_attribute_value(AttributeNames::value);
+        if (!result.has_value())
+            result = submitter->get_attribute_value(AttributeNames::value);
 
         // 6. Close the dialog subject with result.
         subject->close(move(result));