<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .active{
            color: red;
        }

        .default{
            font-size: 20px;
            border: 1px gray solid;
        }
    </style>
</head>
<body>
    <div id="app">
        <div :class="['default',isActive?'active':'']">{{value}}</div>
    </div>

    <script src="../js/vue.js"></script>
    <script>
        const vm = new Vue({
            el:'#app',
            data:{
                value:'待我长发及腰',
                isActive: true,
            }
        })
    </script>


</body>
</html>