Examine source code of CheckiO

{
  "manifest_version": 2,
  "name": "CheckiO",
  "version": "1.2.4",
  "description": "CheckiO is the game for coders https://checkio.org/",
  "homepage_url": "https://checkio.org",
  "background": {
    "scripts": [
      "main.js"
    ]
  },
  "content_security_policy": "script-src 'self'; object-src 'self'",
  "content_scripts": [
    {
      "run_at": "document_end",
      "all_frames": false,
      "matches": [
        "https://checkio.org/*",
        "https://*.checkio.org/*",
        "https://empireofcode.com/*",
        "https://*.empireofcode.com/*"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "permissions": [
    "tabs",
    "management",
    "nativeMessaging"
  ],
  "icons": {
    "16": "icon16.png",
    "128": "icon128.png"
  },
  "applications": {
    "gecko": {
      "id": "{c7e3ccfd-0398-411b-8607-fa4ae25b4cd3}",
      "strict_min_version": "50.0"
    }
  }
}