class MyElement extends HTMLElement {

  get content() {

    return this.getAttribute('content');

  }

​

  set content(val) {

    this.setAttribute('content', val);

  }

}

​

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

​

function customTag(tagName, fn){

  Array

    .from(document.getElementsByTagName(tagName))

    .forEach(fn);

}

function myElementHandler(element) {

  element.textContent = element.content;

}

​

customTag('my-element', myElementHandler);