Priority of required vs default

This is a question for any element, but particularly for a plugin (e.g. http://sdformat.org/spec?ver=1.8&elem=world#gui_plugin ).

If both a default value (e.g., `__default__`) is present and the attribute is set to `required=1` which one of these is taking priority? Is this a valid element of SDFormat:

```

```

Or should the default be ignored? (makes perhaps more sense here, but I'm not sure about the general case)