Examine source code of Pixel Ruler

{
  "manifest_version": 2,
  "name": "Pixel Ruler",
  "version": "1.0",
  "description": "Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.",
  "permissions": [
    "activeTab"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "commands": {
    "pixel-ruler": {
      "suggested_key": {
        "default": "Alt+Shift+M"
      },
      "description": "Pixel ruler tool"
    }
  },
  "browser_action": {
    "default_title": "Pixel Ruler",
    "default_icon": {
      "64": "icon64.png",
      "128": "icon128.png"
    }
  },
  "icons": {
    "64": "icon64.png",
    "128": "icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ede669d7-29e4-403f-8384-428976c8178d}",
      "strict_min_version": "102.0"
    }
  }
}