Examine source code of PX: Viewport Dimensions

{
  "manifest_version": 2,
  "name": "PX: Viewport Dimensions",
  "version": "2.0",
  "description": "Displays the viewport dimensions when a browser window is resized.",
  "applications": {
    "gecko": {
      "id": "lukempeters@gmail.com"
    }
  },
  "icons": {
    "32": "icons/px-32.png",
    "48": "icons/px-48.png",
    "64": "icons/px-64.png",
    "96": "icons/px-96.png",
    "128": "icons/px-128.png"
  },
  "permissions": [
    "storage"
  ],
  "browser_action": {
    "default_icon": "icons/px-32.png",
    "default_title": "Viewport Dimensions",
    "default_popup": "popup/settings.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "viewport-dimensions.js"
      ],
      "css": [
        "css/viewport-dimensions.css"
      ]
    }
  ]
}