Examine source code of CSS Size converter

{
  "manifest_version": 2,
  "name": "CSS Size converter",
  "description": "A tool to help you calculate em and rem value of given px",
  "version": "1.0.0",
  "icons": {
    "128": "icon_128.png"
  },
  "browser_action": {
    "default_icon": "icon_128.png",
    "default_popup": "panel.html"
  }
}