Four levels of DOM

Level 0 The application supports an intermediate DOM, which existed before the creation of DOM Level 1. Examples include the DHTML Object Model or the Netscape intermediate DOM. Level 0 is not a formal specification published by the W3C but rather a shorthand that refers to what existed before the standardization process.
Level 1 Navigation of DOM (HTML and XML) document (tree structure) and content manipulation (includes adding elements). HTML-specific elements are included as well.
Level 2 XML namespace support, filtered views and events.
Level 3 Consists of six different specifications:

DOM Level 3 Core
DOM Level 3 Load and Save
DOM Level 3 XPath
DOM Level 3 Views and Formatting
DOM Level 3 Requirements and
DOM Level 3 Validation, which further enhances the DOM