i'm reading through Dive into HTML5: Offline web applications: Cache manifest

... but it should be offered using the content type text/cache-manifest. If you're running an Apache-based web server, you are able to most likely just put an AddType directive within the .htaccess file at the bottom of the web directory

AddType text/cache-manifest .manifest

basically dont add this what's going to happen? may be the content type essential? used to do searching for AddType in httpd.conf. however i dont see content-type for say html, js, images etc. it dont appear everything important in the end?

The chances are without having this content type set properly the browsers will not address it like a manifest file, like the well-known Opera problem where stylesheets get overlooked if offered having a plain text MIME type.

This content kinds of your Apache server are possibly not in httpd.conf, they're usually saved in another configuration file mime.types (automatically, though it may be transformed).