LibWeb: Implement HTMLOrSVGElement.tabIndex

Author: https://github.com/Lubrsi
Commit: 1473bc9169
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -164,4 +164,11 @@ void run_iframe_load_event_steps(HTML::HTMLIFrameElement& element)
     // FIXME: 6. Unset childDocument's iframe load in progress flag.
 }
 
+// https://html.spec.whatwg.org/multipage/interaction.html#dom-tabindex
+i32 HTMLIFrameElement::default_tab_index_value() const
+{
+    // See the base function for the spec comments.
+    return 0;
+}
+
 }