!!! Overview [1] [2]
[{$pagename}] are part of a [Software Architecture Model] referred to as [Progressive Web Application] and is used used by [Service  Workers] for rapid loading Basic User Interface or "shell". 

[{$pagename}] provides an initial static frame, a layout or [architecture] into which content can be loaded progressively as well as dynamically, allowing users to engage with the [Progressive Web Application] despite varying degrees of [Web] connectivity. Technically, the shell is an [Application] stored locally in the [browser] [cache] of the [Mobile Device] or [User-agent].

[{$pagename}] separates the core [application] infrastructure and [User Interface] from the [data]. All of the [User Interface] and infrastructure is [cached|cache] locally using a [Service Worker] so that on subsequent loads, the [Progressive Web Application] only needs to retrieve the necessary [data], instead of having to load everything.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [What are Progressive Web Apps?|http://blog.ionic.io/what-is-a-progressive-web-app/|target='_blank'] - based on information obtained 2017-08-02- 
* [#2] - [Progressive_web_app|Wikipedia:Progressive_web_app|target='_blank'] - based on information obtained 2017-08-02-