The Application Cache, also known as AppCache, is deprecated.
How the Lighthouse Application Cache audit fails
Lighthouse flags pages that use the Application Cache:
This audit fails when Lighthouse finds a reference to
the Application Cache manifest in a page's <html> tag.
For example, this markup causes the audit to fail:
<html manifest="example.appcache">
...
</html>
Use the Cache API instead of the Application Cache
To pass this audit, remove the manifest from your page, and use the Cache API via a service worker instead.
To migrate from the Application Cache to service workers, consider using the sw-appcache-behavior library. This library generates a service-worker-based implementation of the behavior defined in an Application Cache manifest.
See the Current page does not respond with a 200 when offline post for more information about using service workers to make your site work offline.