index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>TodoList</title>
    <link rel="stylesheet" href="style.css">
    <script src="https://cn.vuejs.org/js/vue.js"></script>
</head>
<body>
    
    <div id="app">
        <input type="text" placeholder="What todo" v-model="todo" @keyup.enter="add(todo)">
        <ul>
            <li v-for="item in list" >
                <span :class="{ crossline : item.done}">{{item.content}}</span>
                <a href="#" @click.prevent="remove(item)">Remove</a>
                <input type="checkbox" v-model="item.done">
            </li>
        </ul>
        <button type="button" @click.prevent="removeall()">Delete all</button>
        <button type="button" @click.prevent="checkall()">All Done</button>
        <button type="button" @click.prevent="uncheckall()">All Undone</button>
    <script src="app.js"></script>
</body>
</html>