At a minimum, there are three core technical requirements to be a PWA. A website must have a web manifest file, be served using HTTPS, and must register a service worker with a fetch event handler. You will dive deeper into each one of these requirements in future chapters.

The web manifest drives the Add to Homescreen experience. HTTPS provides a layer of security and trust between your application and the browser. The service worker provides the extensible backbone for event-driven functionality to execute on a separate thread from the user interface.

A PWA should also use an application shell or common HTML and CSS. This is the most common application of Chrome, which is used on just about every page on the site. If you have any experience with single page applications, you should understand what an application shell is.

A typical application shell typically contains a header, a main content area, and a footer. Of course, this can vary by application and site. The 2048 game differs because there is only one web page:

Application shells are common-place in single page applications because they dynamically render markup and data in the browser. This does not need to be the case with a PWA. The reason single page applications are so popular is their ability to create a more native-like transition experience because there is no request delay when a new page is requested.

Since a PWA can be cached locally, this does not mean you need a true application shell. If the application utilizes a cache first strategy, pages can load in a matter of milliseconds, often less than 100. This is perceived as instant by the human mind.

This does not mean you should not identify an application shell. Server and build rendering engines can use the application shell and an array of layouts to create server hosted markups. You will be exposed to this so that you can work as we build the photo gallery and podcast application.