The HTML element is the generic container for flow content. It has no effect on the content or layout until styled in some way using CSS (e.g. styling is directly applied to it, or some kind of layout model like Flexbox is applied to its parent element).| MDN Web Docs
The Element.attachShadow() method attaches a shadow DOM tree to the specified element and returns a reference to its ShadowRoot.| MDN Web Docs
The HTMLElement interface represents any HTML element. Some elements directly implement this interface, while others implement it via an interface that inherits it.| MDN Web Docs
One of the key features of web components is the ability to create custom elements: that is, HTML elements whose behavior is defined by the web developer, that extend the set of elements available in the browser.| MDN Web Docs