<span>표준 요소</span>
<red-span>비표준 요소</red-span>
class RedSpan extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: 'open' }); 
    this.shadowRoot.innerHTML = `
      <style>
        span { 
          color: red;
        }
      </style>
      <span>
        <slot/>
      </span>
    `;
  }
}

customElements.define('red-span', RedSpan);

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External JavaScript

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