HTML5 Application Cache.:
The manifest file contains three parts CACHE MANIFEST, NETWORK and FALLBACK. You can create this simple text file using notepad with extension
List all the files what exactly you want to be cached under the browser mainfest.
Sometimes the webpage strongly required internet access to call APIs, in such a case we need to keep resource urls under this header.
If you want to display some special no internet page like 404, keep the file or url under this header.
Few servers are not supporting file type extension, include this file for better result.
Application Cache Clear in Chrome