JSFiddle - React, Tailwind, and code Playground

HTML

<a href="">Основы html<span class="mli" id="adm">20</span></a>

<a href="">Java Script<span class="mli" id="bfg">11</span></a>

  <button id='button' value='Отправить на сервер'>Показать во фрейме</button>
  
  <iframe id='frame' style='display:none'></iframe>

JavaScript

button.addEventListener('click',handler);


function handler() {

	var sumNum = +(adm.innerHTML)+(+(bfg.innerHTML));
  
  frame.contentWindow.document.documentElement.innerHTML=sumNum;
  
  frame.style.display='block';

}