Examine source code of Focus Finder

{
  "manifest_version": 2,
  "name": "Focus Finder",
  "version": "1.4",
  "description": "Globally highlights the currently focused or hovered input field, button or action item. This helps visually navigating and observing the structure of a website.",
  "icons": {
    "48": "icons/target-6x.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "focus-finder.user.js"
      ]
    }
  ]
}