Examine source code of Localhost Detector

{
  "manifest_version": 2,
  "name": "Localhost detector",
  "description": "Detects if you are accessing localhost or production and respectively shows prominent red banner only on localhost.",
  "version": "0.4",
  "icons": {
    "128": "icon.png"
  },
  "content_scripts": [
    {
      "js": [
        "local.js"
      ],
      "css": [
        "local.css"
      ],
      "matches": [
        "*://localhost/*",
        "*://127.0.0.1/*",
        "*://*.local/*"
      ],
      "run_at": "document_end"
    }
  ]
}