JSFiddle - React, Tailwind, and code Playground

JavaScript

function handler() {
    if (this.readyState === 4) {
        alert("OK!\n" + this.responseText);
    }
}
var req = new XMLHttpRequest();
req.open("POST", "/echo/html/", true);
req.onreadystatechange = handler;
req.send("html=123");