This object is used to find out about the DOM implementation in a web browser in the same way that the navigator object tells you about the browser version and its capabilities.
The most useful item belonging to this object is the hasFeature() method. With that you can establsih whether your script is running in an environment that supports the capabilities that you need.
This may not be implemented in all version of the MSIE browser. It has been reported as being undefined in MSIE 5.0 and 5.5 for Windows.