JSFiddle - React, Tailwind, and code Playground

HTML

<form name= "reg" id="reg" method="POST" action="user.php" onsubmit="return validate()">

    <label for="first">First Name: </label>
    <input id="first" name="first" type="text" value="">

    <input type="submit">Register</button>
</form>

JavaScript

function validate(){
    var formIsValid = true;
    var first=document.forms["reg"]["first"];
    if(first.value == null || first.value == ""){
        first.style.borderColor = "red"; //NOT WORKING
        formIsValid = false;    
    }

    return formIsValid;

}