GUI specification files may be written using XMLmind XML Editor^[2] or using a text editor.

If you use a text editor, do not forget to validate your GUI specification against its schema. After downloading and installing the add-on called "XMLmind XML Editor Configuration Pack" using Options → Install Add-ons, this schema is found in XXE_user_preferences_dir/addon/xxe_config_pack/gui/xsd/gui.xsd. Linux example:

~/.xxe11/addon$ /opt/xxe/bin/xmltool validate \
    -s ~/.xxe11/addon/xxe_config_pack/gui/xsd/gui.xsd customize.xxe_gui