Hey Kelly, you can feature detect the existence of the various web components standards client side using a javascript snippet. I covered it during part of my talk at Chrome Dev Summit:
https://www.youtube.com/watch?v=kV0hgdMpH28&feature=youtu.be&t=20m3s
Basically it checks to see if the all the web component technologies are supported and if not, it loads webcomponents.js. But you could do more in that handler like display a message. Another thing to keep in mind is that Web Components are not polyfillable on really old browsers like IE 8 and the old stock Android browser. So if you have a high percentage of users coming from those browsers (and you should be able to check that with Google Analytics) then you could consider trying to detect that server side and send down a different page. That's what GitHub does