The apple-mobile-web-app-capable metatag defines wether a website/webapp which was added to the homescreen can run in fullscreen or not.
Apparently this does not work anymore and is therefor marked as Unsupported.
Read more about Apple's Webapps at Apple's official documentation.
<meta name="apple-mobile-web-app-capable" content="yes">
content can also be
false, but the meta-tag is obsolete then.
In the WHATWG WikiUp to date: v2.3.3