Web Components（カスタム要素 + Shadow DOM）とは何ですか？

class UserCard extends HTMLElement {
  connectedCallback() {            // lifecycle: runs when added to the page
    const shadow = this.attachShadow({ mode: "open" }); // create shadow DOM
    shadow.innerHTML = `
      <style> h2 { color: blue; } </style>
      <h2>${this.getAttribute("name")}</h2>
      <slot></slot>
    `;
  }
}
customElements.define("user-card", UserCard); // name MUST contain a hyphen

<user-card name="Ann">Extra content goes in the slot</user-card>

/* page CSS */ h2 { color: red; } /* does NOT affect the <h2> inside user-card */

connectedCallback()    // added to DOM
disconnectedCallback() // removed from DOM (cleanup)
attributeChangedCallback(name, old, val) // an observed attribute changed
static get observedAttributes() { return ["name"]; }