Examine source code of Signature Viewer

{
  "manifest_version": 2,
  "name": "Signature Viewer",
  "description": "Shows information about embedded XAdES signatures",
  "version": "1.1.2",
  "author": "Chris Groenveld",
  "background": {
    "scripts": [
      "xades.js",
      "bg.js"
    ]
  },
  "icons": {
    "128": "doc128tick.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "check_signature.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_action": {
    "default_icon": {
      "128": "doc128gray.png"
    },
    "default_title": "Signature Viewer"
  }
}