The Demo form allows you to experiment on one file at a time, to test and tune the import settings, or see how viable it is to import existing web content.
Enter one HTML filepath or URL to process as an import. The retrieved data will not automatically become part of the site unless you confirm it, it's just a demo of what data would be extracted on a simple import.
The given file will be passed through the currently configured import process and shown as a node-edit form, displaying all the deduced content in the appropriate fields.
If you see too much of the page in the 'body' area (nested navbars and layout) then the XSL import template or selector needs to be made more specific. If you see none, or not enough content in the body area, the template or selector needs to be adjusted to encompass the text correctly.
If you get an error or no result, the input HTML is probably too invalid to work with.
Single demo imports do not have the full context information to work with, so the menu or URL alias (and internal relinking) shown may not be representative of the real result.