Page 298 of "ASP.NET Core 2 and Angular 5" by Valerio De Sanctis suggests that the Shadow DOM, HTML Templates, custom elements, and HTML Imports are the four Web Components standards.

The first three are covered elsewhere on this blog. This suggests that "HTML Imports is intended to be the packaging mechanism for web components, but you can also use HTML Imports by itself." and offers as an example:

<link rel="import" href="myfile.html">