There are three interfaces in the org.w3c.dom.ranges package, e.g.

$ javap -J-showversion org.w3c.dom.ranges.DocumentRange
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

Compiled from "DocumentRange.java"
public interface org.w3c.dom.ranges.DocumentRange{
    public abstract org.w3c.dom.ranges.Range createRange();
}

And src.zip includes the corresponding source files (DocumentRange.java, Range.java, and RangeException.java).

Yet

http://java.sun.com/javase/6/docs/api/overview-summary.html

does not mention the existence of this package. It should, I guess.

There seem to be other undocumented DOM packages, too: org.w3c.dom.traversal, .css, .html, .stylesheets, .views, .xpath.