Complex sentence: Therefore, when connection is interrupted, the application is no longer usable but if it uses HTML5 API's such as Offline Web application caching, it can be downloaded and installed locally, for offline use.
Split into simpler sentences: Therefore, when connection is interrupted, the application is no longer usable. However, if it uses HTML5 API's such as Offline Web application caching, it can be downloaded and installed locally, for offline use.