Add rule(s) to check the usage of the Web App Manifest file

[alrra]

• Check if the manifest is specified and exists (done in Add manifest-exists rule and make other improvements #54)
• Check if the manifest file is valid (partially done in: Add rule to check if the web app manifest content is valid #56)
• Check if the file is not sent with the application/manifest+json media type (will be solved by Add rule to check the usage of the Content-Type HTTP response header #141)
• Check if the file extension is webmanifest (done in Add rule to check the web app manifest extension #39)

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See: https://www.w3.org/TR/appmanifest/

[alrra]

What browsers need to display the app install banner (add to home screen banner) related to the web app manifest file:

Browser	display set to standalone or fullscreen	At least a 144x144 PNG	name	short_name	start_url
Chrome for Android (see code)	yes	yes	yes	yes	yes
Opera for Android	no (?)	yes	yes	yes	yes
Samsung Internet	yes	yes	yes	yes	yes

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See also:

• https://infrequently.org/2016/09/what-exactly-makes-something-a-progressive-web-app/
• https://www.youtube.com/watch?v=m-sCdS0sQO8&t=12m31s

[alrra]

For icons, see also: https://www.youtube.com/watch?v=TB-K6OniF68.

[alrra]

Firefox for Android:

• https://hacks.mozilla.org/2017/10/progressive-web-apps-firefox-android/
• https://www.youtube.com/watch?v=_sLa0qhuqcA&t=12m40s

[alrra]

For Samsung Internet, see: http://developer.samsung.com/internet/android/web-guide.