Identical if/else conditions in gtkplacesview.c
if (scheme && !supported)
gtk_widget_add_css_class (view->address_entry, GTK_STYLE_CLASS_ERROR);
else
gtk_widget_add_css_class (view->address_entry, GTK_STYLE_CLASS_ERROR);
https://gitlab.gnome.org/GNOME/gtk/-/blob/master/gtk/gtkplacesview.c#L1890-1893