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/HTMLButtonElement.cpp

@@ -86,4 +86,11 @@ void HTMLButtonElement::set_type(String const& type)
     MUST(set_attribute(HTML::AttributeNames::type, type));
 }
 
+// https://html.spec.whatwg.org/multipage/interaction.html#dom-tabindex
+i32 HTMLButtonElement::default_tab_index_value() const
+{
+    // See the base function for the spec comments.
+    return 0;
+}
+
 }