A feature of HTML 5 based applications is the ability to ensure that applications can still run even if internet connectivity is not present. How to do this is covered on the W3.org website.
A requirement of offline access is the creation of the offline cache manifest file. This manifest file is listed in the HTML tag on the page as such:
And a page is saved to the web server with the same name (offline.appcache in this example). This .appcache file follows the conventions described in the above W3.org web page, but this page needs to be served from the web server with a specific MIME type (text/cache-manifest). If the web server is IIS 5.0 or later then it will only serve content that has been listed as a valid MIME type in Windows. If you used a shared hosted webserver then making that change is probably impossible – so from IIS 7.0 or later you can add your own MIME type in the admin UI or modify the web.config file in the root of your web server to add this MIME type. This is just a text file that you upload and so requires no access to the IIS admin application (again, typically something you do not get with shared hosted web server).
Note: In the example given below, the web.config file changes two properties. If you have an existing web.config file then merge these changes into your file and do not replace your file.
The web.config file needs to be as follows:
xml version="1.0" encoding="UTF-8"?>
<mimeMap fileExtension=".appcache" mimeType="text/cache-manifest" />
<clientCache cacheControlMode="DisableCache" />
The two changes set in this web.config file are, firstly, mimeMap in the staticContent section of system.webServer. This adds the .appcache extension as text/cache-manifest. The second change is clientCache in staticContent section of system.webServer (but this time in a location section, limiting the effect of the setting to the named file – offline.appcache). This change stops the web server or client from caching the page, ensuring that the web server always serves the latest copy of the page.
Upload web.config and your appcache manifest file, along with any page that needs to be viewed offline (or indeed any page that you want to speed up loading for, by causing the pages to be cached on the client) and check that when you browse to the .appcache file directly in a HTML 5 aware browser it is visible. If you get a 404 error on this page then you have not set the MIME type or uploaded the correct web.config file.