Notice

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.

Service Worker API

Validate by URI

Validate a document online:

Validate by File Upload

Upload a document for validation:

Validate by Direct Input

Enter the manifest to validate:

Check cache manifest files for web application offline storage

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:

API

This validation service can also be used by external tools. Either provide an URI to your manifest or the file's content as a string (GET and POST are supported for calling). The API either returns a JSON string or a JavaScript function for JSONP, when you provide the optional callback parameter.

Try out the interactive Cache Manifest Validator API documentation.