v1 tabs tightly couple to their v1 Cache; v2 tabs tightly couple to their v2 Cache. This tight coupling makes them “application caches.” The app must be completely shut down (all tabs closed) in order to upgrade atomically to the newest cache of code.

Service Workers are the hot new thing in web APIs. They’re designed to be like the HTML5 Application Cache, but without being objectionable.

Service Workers are also helpful for offline caching and performance. You can write code to cache your application shell to work offline and populate its content using JavaScript.