Examine source code of Header Checker

{
  "manifest_version": 2,
  "name": "Header Checker",
  "version": "1.0",
  "description": "Free Header Tag Checker Tool (H1 up to H6)",
  "icons": {
    "48": "01.png",
    "96": "02.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://*/*",
        "http://*/*"
      ],
      "js": [
        "index.js"
      ],
      "css": [
        "index.css"
      ]
    }
  ]
}