We need Cache API to improve a performance in client side.
If client developer uses the API, most of static pages should be cached automatically.
So, we might need the following requirements for the API.

• Developer doesn't (or shouldn't) know ServiceWorker concept.
• Developer just includes and uses the API in main thread.
  (In other words, there is no reason to handle the API in worker thread).
• At least, should caches all static pages.