Appcache has been
marked as deprecated since Firefox 44
and will be
removed, Chrome is likely to follow soon.
It will also be removed from
the future HTML spec.
Now it's a good time to learn about Service Workers, which are providing a better programmatic interface for network requests and more granular control over handling offline resources.
The most complete method is to upload the manifest file to your server and validate it by URI. This will also check the availability of all resources listed in your manifest – if even one of them fails to download properly, the entire process of caching your offline web application will fail.
To learn more about offline web applications and cache manifest files, check the following resources:
Try out the interactive Cache Manifest Validator API documentation.