The level 3 DOM is an enhancement on the level 2 support. It introduces additional properties and interfaces for document objects.
The standard is composed of the following modules:
DOM core functionality
Traversal and range specification
Content model and archiving
The DOM level specification has grown again although not as much as it did at the level 2 version. Some changes to the Core and Events modules have taken place and a new module has been introduced to support content models and the ability to export and import documents between implementations.
Level 3 standardization is still very much a work in progress and as a whole the DOM standardization process is somewhat held back by work in other groups to do with XML and internationalization among other things. Progress is still being made and although there is much to do, many aspects of the DOM standard are now reasonably well defined and not likely to change.
It still remains for the browser manufacturers to catch up with the standard and in particular to use the same naming conventions for object classes instead of continually inventing their own.