Application Cache, also known as AppCache, is deprecated. Lighthouse flags documents that use AppCache:
How this audit fails
Lighthouse fails this audit when it finds a reference to the AppCache manifest on the document's html tag. For example, this markup in your document causes the audit to fail:
Each Best Practices audit is weighted equally in the Lighthouse Best Practices Score. Learn more in The Best Practices score.
Use the Cache API instead of AppCache
To pass this audit, remove the manifest from your document, and use the service worker Cache API instead.
To help migrate from AppCache to service workers, consider using the sw-appcache-behavior library. This library generates a service-worker-based implementation of the behavior defined in an AppCache manifest.
See the URL Responds With a 200 When Offline audit reference for more resources on using service workers to make your site work offline.
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