Custom Elements (V1)
One of the key features of the Web Components system, custom elements allow new HTML tags to be defined.
Deprecated method of including and reusing HTML documents in other HTML documents. Superseded by ES modules.
Shadow DOM (V1)
Method of establishing and maintaining functional boundaries between DOM trees and how these trees interact with each other within a document, thus enabling better functional encapsulation within the DOM & CSS.
Method of declaring a portion of reusable markup that is parsed but not rendered until cloned.