Manifest.json はプログレッシブ Web アプリケーションに必要です。

{
    "background_color": "#fff",
    "display": "standalone",
    "description": "Awesome application portal to Healing Services",
    "icons": [
        { "src": "192.png", "sizes": "192x192", "type": "image/png" },
        { "src": "512.png", "sizes": "512x512", "type": "image/png" }
    ],
    "name": "Holy Spirit Healing Services",
    "short_name": "The Holy Spirit",
    "start_url": "./healingservices.html"
}