Examine source code of HSTS Checker

{
  "manifest_version": 2,
  "name": "HSTS Checker",
  "version": "1.0",
  "description": "Notifies the user if the current webpage includes an HTTP Strict Transport Security (HSTS) header.",
  "permissions": [
    "tabs",
    "webRequest",
    "webNavigation"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "hsts_checker_extension@gmail.com"
    }
  }
}