JSFiddle - React, Tailwind, and code Playground

HTML

CIF <input type="text" id="txtCif" onkeyup="validarCif(this.value)" />
<br />
Validando este CIF: <span id="validando" class="azulito">....</span>

CSS

.azulito{
    color:blue;
    font-weight:bold;    
}

JavaScript

function validarCif( cif ){
    document.getElementById("validando").innerHTML=cif;
}