LibWeb: Return true for is_focusable() elements with non-null tabindex

This change implements the requirements stated in the HTML spec at
https://html.spec.whatwg.org/multipage/interaction.html#tabindex-value
that UAs must allow HTML elements with non-null tabindex values to be
considered as focusable areas.

Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -92,7 +92,7 @@ void HTMLElement::set_dir(String const& dir)
 
 bool HTMLElement::is_focusable() const
 {
-    return is_editing_host();
+    return is_editing_host() || get_attribute(HTML::AttributeNames::tabindex).has_value();
 }
 
 // https://html.spec.whatwg.org/multipage/interaction.html#dom-iscontenteditable