What file type is essential for a web application to function offline?

The file type that is essential for a web application to function offline is a manifest file. The purpose of a manifest file in the context of web applications is to provide a list of resources that the browser should cache, allowing the application to be accessible without an internet connection. This is part of the Application Cache feature in HTML5, which enables developers to define which files (such as HTML, CSS, JavaScript, and images) should be downloaded and stored locally on the user's device.

When the application is launched, the browser checks the manifest file, verifies the resources to be cached, and makes them available for offline access. This ensures that users can interact with the web application even when they are not connected to the internet, enhancing usability and accessibility.

Manifest files essentially serve as a blueprint for how the browser should handle caching, enabling the offline capabilities that are essential for seamless web application performance when connectivity is compromised.