Appcache has been
marked as deprecated in favor of the Service Worker API since Firefox 44
and will be
removed in the future.
Likewise Chrome will disable appcache over HTTP soon, as a first step to deprecating this API.
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.