JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="someinput" onkeyup="validateinput()" />

CSS

.red
{
    background: red;
}
.green
{
    background: green;
}

JavaScript

function validateinput()
{
    var inputbox = document.getElementById("someinput");
    
    if(isNaN(parseFloat(inputbox.value)))
    {
        inputbox.className = "red";
    }
    else
    {
        inputbox.className = "green";
    }
}