Q:

What are DOM levels?

I've heard DOM level 2 and DOM level 3 mentioned in previous posts.  Are these abstract constructs like the ISO OSI model which has layers 1-7 but are only loosely followed?

A:

DOM Levels are the versions of the specification for defining how the Document Object Model should work, similarly to how we have HTML4, HTML5, and CSS2.1 specifications.
The most recent spec is DOM Level 3, published in April 2004.
Additionally, there are pieces of the DOM spec that vendors can choose to implement, such as Core, HTML, and XML, as well as the event model. Depending on what is being built (a DOM parser, web browser layout engine, or javascript engine), the vendor may choose to implement some or all of the spec. Most modern web browsers implement all of the Level 3 spec.

A:

DOM Levels are essentially versions.
DOM Level 1 defines the core elements of the Document Object Model.
DOM Level 2 extends those elements and adds events.
DOM Level 3 extends DOM lvl 2 and adds more elements and events.
Each new level of the DOM adds or changes specific sets of features. When browsers are said to be DOM Level X compliant developers can (hopefully) assume that the browser correctly handles the specified DOM api calls.

A:

I know this question is old, but I'm adding this data for any people coming across this post.
The DOM used to be written as a set of levels. That is no longer the case.
These days it is maintained as the DOM Living Standard
See also the DOM Reference for more recent web-developer-focused information.
Source: Here

