webcomponentsjs 
The popular WebComponents polyfill in a nice, frequently updated NPM package.
Usage
Install webcomponentsjs as a dependency:
npm install webcomponentsjs
The polyfill traditionally comes in two standard sizes and this version adds an additional one
that is significantly smaller!
full
includes the Custom Elements, HTML Imports, Shadow DOM and Templates polyfills.
(260kb)
lite
includes everything from the full polyfill except for the Shadow DOM.
(79kb)
micro
Includes polyfills for only Custom Elements and Templates.
NOTE: The micro package doesn't include the polyfill for the [URL()] (https://developer.mozilla.org/en-US/docs/Web/API/URL/URL) object
because they were only needed for the HTMLImports polyfill.
(40kb)
To use it, import it as a dependency from your code:
import "webcomponentsjs"
If you want to load the lite or micro versions, use:
import "webcomponentsjs/lite"
or
import "webcomponentsjs/micro"
That's it!
