Natural View is an Internet Explorer window showing your file. This is feature for XML documents.

It is very useful for .XHTML files, especially for dual-monitor computers. I recommend open Natural View in the second monitor on full screen. You edit a text, Program Tree Editor updates the tree and at once updates the content of Natural View. So you will see how change the view of your XHTML page without any efforts.

See also: When is a tree updated?