Speaking of Web Components, this script lists the custom elements registered on a browser's current page.

(
  [...new Set([].map.call(
    document.querySelectorAll('*'), 
    el => el.nodeName.toLowerCase()))
  ].filter(
    customElements.get.bind(customElements)
  )
  .sort()
)

You can run it in your browser's JavaScript console to analyze the current page.

That's how you can find for example that the gist page which contains this script uses no less than 28 custom elements - GitHub is serious about Web Compoments!