A Custom Element is a developer defined HTML tag. These elements are the foundation of Web Components and can be used to create any sort of UI. To learn more about Custom Elements check out the guide on Web Fundamentals.
Shadow DOM introduces scoped CSS and DOM to the web platform. It lets developers write encapsulated UI components which can be used in any application. To learn more about Shadow DOM check out the guide on Web Fundamentals.
Every Custom Element has a set of built-in lifecycle callbacks, or "reactions" that are called when the element is added/removed from the page, or has an attribute mutated. To learn more about Custom Element lifecycle callbacks check out the guide on Web Fundamentals.
Mutation Observers are a web platform API to detect and react to change to the DOM tree. If an element is observed by a MutationObserver, events like appending a new element or removing and existing one will trigger a function to execute. For more details, checkout out the guide on WebFundamentals.
FLIP is a technique to set up high-performance animations using CSS transforms. To avoid janking animations, start and end position are evaluated during the setup so that the animation doesn't have to do any expensive calculations. For a detailed introduction, check out Paul Lewis' guide on his blog.