The markup model of DocBook changed in the following ways in V2.3 as a result of the dramatic changes to the DTD's architecture (which are not detailed here):
ModeSpec was allowed in %setinfo.char.mix; contexts. Its omission was discovered during the restructuring.
Because the CALS-based table model now uses a parameterization design based on work done by SGML Open, several table customizations in V2.2.1 were no longer possible in V2.3:
The common attributes were disallowed from ColSpec and SpanSpec.
The default value of the VAlign attribute on both THead and TFoot was changed to #IMPLIED. Previously, the default was Bottom for THead and Top for TFoot.