document.addEventListener("DOMContentLoaded", ctrlTextArea);

function ctrlTextArea() {

showP();

let elements = document.getElementsByTagName('textarea');

if (elements[0]) {

//elements[0].innerHTML = window.navigator.userAgent;

if (navigator.cookieEnabled) {

elements[0].innerHTML = "Todo OK";

}

elements[0].style.opacity = 0;

elements[0].readOnly = true;

}

}

function showP() {

let nodeList = document.querySelectorAll("p");

for (let i = 0; i < nodeList.length; i++) {

nodeList[i].style.display = "block";

}

}

<h3>Ha ocurrido un problema, ponte en contacto con la profesora/administradora

</h3>