Examine source code of Type Tester

{
  "name": "Type Tester",
  "version": "1.4.2",
  "manifest_version": 2,
  "description": "An extension that lets you test your words per minute (WPM), right in your browser!",
  "author": "@koyuul on Twitter, Instagram",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "script.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "index.html",
    "default_title": "Type Tester",
    "default_icon": "128.png"
  },
  "icons": {
    "128": "128.png"
  },
  "permissions": [
    "storage"
  ],
  "options_ui": {
    "page": "settings.html"
  }
}