WebComponents

Web Components are a set of web platform APIs that enable developers to create reusable, encapsulated, and interoperable custom elements. The core technologies are Custom Elements, Shadow DOM, and HTML Templates. HTML Imports, once proposed as a means to include HTML documents, is now deprecated. Together, these APIs let developers define new HTML tags with their own behavior and styling while keeping them isolated from the rest of the page.

Custom Elements provide a way to define a new element type by creating a class that extends HTMLElement and registering it with the customElements.define method. The element lifecycle is managed through callbacks such as connectedCallback, disconnectedCallback, attributeChangedCallback, and adoptedCallback. Shadow DOM creates an encapsulated subtree via attachShadow, supporting open or closed modes, which confines styles and markup to the component. HTML Templates offer inert markup that can be cloned for each instance, enabling efficient rendering; slots inside the shadow tree allow external content to be projected into the component.

Web Components are designed to be framework-agnostic and can be used alongside libraries or plain JavaScript. They are commonly supported in modern browsers, and can be enhanced with polyfills for older environments. Many design systems and UI libraries build components as Web Components to enable reuse, consistency, and interoperability across projects.

History and status: Web Components were standardized through the W3C and WHATWG efforts and have achieved broad support in modern browsers. They offer a stable foundation for reusable widgets, especially in design systems. Considerations include accessibility, performance of shadow DOM, and the balance between encapsulation and global styling.