Some properties documented as required, such as labels on wxStaticText or wxCheckBox or the size of wxSizer spacers, are frequently omitted in practice. Rather than strictly requiring them, forcing the markup to include dummy empty elements, loosen the restriction in the spec and make them optional. Nothing changes about actual acceptance of files by wxXmlResource: it would already silently deal with the lack of these properties. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74924 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775