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
The MutationObserver interface provides the ability to watch for changes being made to the DOM tree. It is designed as a replacement for the older Mutation Events feature, which was part of the DOM3 Events specification.| MDN Web Docs