JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input type="text" name="firstname" />
    <input type="submit">
</form>

CSS

input.invalid{ background-color:#dd1112; }

JavaScript

function validateForm(){
    var inputs = this.getElementsByTagName("input")
    var valid = true;
    for(var i=0, len=inputs.length; i<len; i++){
        if(inputs[i].type == "submit" || inputs[i].type == "button")
            continue;
        if(inputs[i].value == ""){
           inputs[i].className = "invalid";
            valid = false
        }else
            inputs[i].className = "";
    }
    return valid
}
var forms = document.getElementsByTagName("form")
for(var i=0, len=forms.length; i<len; i++){
    forms[i].onsubmit = validateForm
}