ce-example-01.js

const $value = Symbol('value');

class MyElement extends HTMLElement {
  static get observedAttributes() {
    return ['value'];
  }
  
  set value(value) {
    this[$value] = value;
  }
  
  get value() {
    return this[$value];
  }
  
  attributeChangedCallback(attr, oldValue, newValue) {
    if (oldValue !== newValue) {
      this[attr] = newValue;
    }
  }
}

customElements.define('my-element', MyElement);