Examine source code of I am testing

{
  "applications": {
    "gecko": {
      "id": "iamtesting@wildsky.cc",
      "strict_min_version": "51.0"
    }
  },
  "manifest_version": 2,
  "name": "I am testing",
  "description": "Reminder Developers that you are testing.",
  "version": "0.1.0",
  "icons": {
    "128": "icon.png"
  },
  "content_scripts": [
    {
      "css": [
        "testing.css"
      ],
      "matches": [
        "*://localhost/*",
        "*://*.test/*"
      ],
      "run_at": "document_start"
    }
  ]
}