JSFiddle - React, Tailwind, and code Playground

HTML

<div id="person" class='bllu'>
    First Name: <input class='redd' v-model="firstName"><br/>
    Last Name: <input v-model="lastName"><br/>
    <span>Full Name: <div class='redd'>{{fullName}}</div></span><br/>
</div>

CSS

.redd {color: red;}
.bllu {color: blue;}

JavaScript

var demo = new Vue({
    el: '#person',
    data: {
        firstName: 'Taro',
        lastName: 'Yamada',
    },
    computed: {
        fullName: {
            get: function(){
                return this.firstName + ' ' + this.lastName;
            }
        }
    },
})