What is application cache in HTML5?

HTML5 Application Cache (AppCache) is a mechanism that allowed web applications to work offline by loading resources (HTML, CSS, JS, images) into the browser's local cache.

Main components of AppCache:

Manifest File: A text file with the extension .appcache, which lists the resources needed for offline operation. It is specified in the manifest attribute of the <html> tag.
```
<html manifest="example.appcache">
...
</html>
```
Manifest sections:
- CACHE MANIFEST: The mandatory first line.
- CACHE:: List of resources that should be explicitly cached.
- NETWORK:: Resources that always require network access (e.g., API). By default, everything not in CACHE and FALLBACK requires network.
- FALLBACK:: Rules for defining fallback resources in case the main resources are unavailable (e.g., offline page).
Lifecycle: AppCache has a complex lifecycle, including loading the manifest, comparing versions, downloading resources, and activating the new cache.

Pros:

Offline access to the application.
Faster loading for returning users.

Cons:

Complexity in cache management and updates ("hanging" cache).
Less flexible compared to Service Workers.

Current status: AppCache is deprecated and gradually being removed from browsers. For implementing offline functionality and resource caching, it is recommended to use Service Workers.