x-blink.js

class XBlink extends HTMLElement {
  constructor() {
    super();

    this.attachShadow({ mode: 'open' })
      .appendChild(document.createElement('slot'));        
  }

  connectedCallback() {
    this.speed = parseInt(this.getAttribute('speed') || '500');
    this.style.opacity = '1';

    this.interval = setInterval(() => {
      this.style.opacity = this.style.opacity === '1' ? '0' : '1'
    }, this.speed);
  }

  disconnectedCallback() {
    clearInterval(this.interval);
  }
}

customElements.define('x-blink', XBlink);