Examine source code of Viewport Master - Responsive

{
  "manifest_version": 3,
  "name": "ViewportMaster - Responsive Design Tester",
  "version": "1.0",
  "description": "Simulates various screen sizes and devices for responsive design testing",
  "icons": {
    "48": "icons/icon-48.png",
    "96": "icons/icon-96.png"
  },
  "action": {
    "default_icon": {
      "48": "icons/icon-48.png",
      "96": "icons/icon-96.png"
    },
    "default_title": "ViewportMaster",
    "default_popup": "popup/popup.html"
  },
  "permissions": [
    "activeTab",
    "storage"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "web_accessible_resources": [
    {
      "resources": [
        "iframe.html"
      ],
      "matches": [
        "<all_urls>"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "viewportmaster@gmail.com"
    }
  }
}