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Adding Classes

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About the <head>

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CSS

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HTML 

              
                <section class="todoapp" id="todo-section">
			<header class="header">
				<h1>todos</h1>
				<input class="new-todo" placeholder="What needs to be done?" autofocus v-model="newTodo" @keyup.enter="addTodo">
			</header>
			<!-- This section should be hidden by default and shown when there are todos -->
			<section class="main">
				<input class="toggle-all" type="checkbox">
				<label for="toggle-all">Mark all as complete</label>
				<ul class="todo-list">
					<!-- These are here just to show the structure of the list items -->
					<!-- List items should get the class `editing` when editing and `completed` when marked as completed -->

				<todo-list v-for="todo in todos" :todo="todo"></todo-list>
					<!-- <li class="completed">
						<div class="view">
							<input class="toggle" type="checkbox" checked>
							<label>Taste JavaScript</label>
							<button class="destroy"></button>
						</div>
						<input class="edit" value="Create a TodoMVC template">
					</li>
					<li>
						<div class="view">
							<input class="toggle" type="checkbox">
							<label>Buy a unicorn</label>
							<button class="destroy"></button>
						</div>
						<input class="edit" value="Rule the web">
					</li> -->
				</ul>
			</section>
			<!-- This footer should hidden by default and shown when there are todos -->
			<footer class="footer">
				<!-- This should be `0 items left` by default -->
				<span class="todo-count"><strong>0</strong> item left</span>
				<!-- Remove this if you don't implement routing -->
				<ul class="filters">
					<li>
						<a class="selected" href="#/">All</a>
					</li>
					<li>
						<a href="#/active">Active</a>
					</li>
					<li>
						<a href="#/completed">Completed</a>
					</li>
				</ul>
				<!-- Hidden if no completed items are left ↓ -->
				<button class="clear-completed">Clear completed</button>
			</footer>
		</section>
		<footer class="info">
			<p>Double-click to edit a todo</p>
			<!-- Remove the below line ↓ -->
			<p>Template by <a href="http://sindresorhus.com">Sindre Sorhus</a></p>
			<!-- Change this out with your name and url ↓ -->
			<p>Created by <a href="http://todomvc.com">you</a></p>
			<p>Part of <a href="http://todomvc.com">TodoMVC</a></p>
		</footer>
<!-- Write all the templates of the application here -->
		<template id="todo-list">
			<li :class="{completed : todo.completed, editing: isEditing}">
						<div class="view">
							<input class="toggle" type="checkbox" v-model="todo.completed">
							<label @dblClick="enableEditing">{{todo.todo}}</label>
							<button class="destroy" @click="removeTodo(todo)"></button>
						</div>
						<input class="edit" v-todo-focus v-model="todo.todo" @keyup.enter="editTodo(todo)">
			</li>
		</template>
!

CSS 

              
                html,
body {
	margin: 0;
	padding: 0;
}

button {
	margin: 0;
	padding: 0;
	border: 0;
	background: none;
	font-size: 100%;
	vertical-align: baseline;
	font-family: inherit;
	font-weight: inherit;
	color: inherit;
	-webkit-appearance: none;
	appearance: none;
	-webkit-font-smoothing: antialiased;
	-moz-osx-font-smoothing: grayscale;
}

body {
	font: 14px 'Helvetica Neue', Helvetica, Arial, sans-serif;
	line-height: 1.4em;
	background: #f5f5f5;
	color: #4d4d4d;
	min-width: 230px;
	max-width: 550px;
	margin: 0 auto;
	-webkit-font-smoothing: antialiased;
	-moz-osx-font-smoothing: grayscale;
	font-weight: 300;
}

:focus {
	outline: 0;
}

.hidden {
	display: none;
}

.todoapp {
	background: #fff;
	margin: 130px 0 40px 0;
	position: relative;
	box-shadow: 0 2px 4px 0 rgba(0, 0, 0, 0.2),
	            0 25px 50px 0 rgba(0, 0, 0, 0.1);
}

.todoapp input::-webkit-input-placeholder {
	font-style: italic;
	font-weight: 300;
	color: #e6e6e6;
}

.todoapp input::-moz-placeholder {
	font-style: italic;
	font-weight: 300;
	color: #e6e6e6;
}

.todoapp input::input-placeholder {
	font-style: italic;
	font-weight: 300;
	color: #e6e6e6;
}

.todoapp h1 {
	position: absolute;
	top: -155px;
	width: 100%;
	font-size: 100px;
	font-weight: 100;
	text-align: center;
	color: rgba(175, 47, 47, 0.15);
	-webkit-text-rendering: optimizeLegibility;
	-moz-text-rendering: optimizeLegibility;
	text-rendering: optimizeLegibility;
}

.new-todo,
.edit {
	position: relative;
	margin: 0;
	width: 100%;
	font-size: 24px;
	font-family: inherit;
	font-weight: inherit;
	line-height: 1.4em;
	border: 0;
	color: inherit;
	padding: 6px;
	border: 1px solid #999;
	box-shadow: inset 0 -1px 5px 0 rgba(0, 0, 0, 0.2);
	box-sizing: border-box;
	-webkit-font-smoothing: antialiased;
	-moz-osx-font-smoothing: grayscale;
}

.new-todo {
	padding: 16px 16px 16px 60px;
	border: none;
	background: rgba(0, 0, 0, 0.003);
	box-shadow: inset 0 -2px 1px rgba(0,0,0,0.03);
}

.main {
	position: relative;
	z-index: 2;
	border-top: 1px solid #e6e6e6;
}

label[for='toggle-all'] {
	display: none;
}

.toggle-all {
	position: absolute;
	top: -55px;
	left: -12px;
	width: 60px;
	height: 34px;
	text-align: center;
	border: none; /* Mobile Safari */
}

.toggle-all:before {
	content: '❯';
	font-size: 22px;
	color: #e6e6e6;
	padding: 10px 27px 10px 27px;
}

.toggle-all:checked:before {
	color: #737373;
}

.todo-list {
	margin: 0;
	padding: 0;
	list-style: none;
}

.todo-list li {
	position: relative;
	font-size: 24px;
	border-bottom: 1px solid #ededed;
}

.todo-list li:last-child {
	border-bottom: none;
}

.todo-list li.editing {
	border-bottom: none;
	padding: 0;
}

.todo-list li.editing .edit {
	display: block;
	width: 506px;
	padding: 12px 16px;
	margin: 0 0 0 43px;
}

.todo-list li.editing .view {
	display: none;
}

.todo-list li .toggle {
	text-align: center;
	width: 40px;
	/* auto, since non-WebKit browsers doesn't support input styling */
	height: auto;
	position: absolute;
	top: 0;
	bottom: 0;
	margin: auto 0;
	border: none; /* Mobile Safari */
	-webkit-appearance: none;
	appearance: none;
}

.todo-list li .toggle:after {
	content: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="40" height="40" viewBox="-10 -18 100 135"><circle cx="50" cy="50" r="50" fill="none" stroke="#ededed" stroke-width="3"/></svg>');
}

.todo-list li .toggle:checked:after {
	content: url('data:image/svg+xml;utf8,<svg xmlns="http://www.w3.org/2000/svg" width="40" height="40" viewBox="-10 -18 100 135"><circle cx="50" cy="50" r="50" fill="none" stroke="#bddad5" stroke-width="3"/><path fill="#5dc2af" d="M72 25L42 71 27 56l-4 4 20 20 34-52z"/></svg>');
}

.todo-list li label {
	word-break: break-all;
	padding: 15px 60px 15px 15px;
	margin-left: 45px;
	display: block;
	line-height: 1.2;
	transition: color 0.4s;
}

.todo-list li.completed label {
	color: #d9d9d9;
	text-decoration: line-through;
}

.todo-list li .destroy {
	display: none;
	position: absolute;
	top: 0;
	right: 10px;
	bottom: 0;
	width: 40px;
	height: 40px;
	margin: auto 0;
	font-size: 30px;
	color: #cc9a9a;
	margin-bottom: 11px;
	transition: color 0.2s ease-out;
}

.todo-list li .destroy:hover {
	color: #af5b5e;
}

.todo-list li .destroy:after {
	content: '×';
}

.todo-list li:hover .destroy {
	display: block;
}

.todo-list li .edit {
	display: none;
}

.todo-list li.editing:last-child {
	margin-bottom: -1px;
}

.footer {
	color: #777;
	padding: 10px 15px;
	height: 20px;
	text-align: center;
	border-top: 1px solid #e6e6e6;
}

.footer:before {
	content: '';
	position: absolute;
	right: 0;
	bottom: 0;
	left: 0;
	height: 50px;
	overflow: hidden;
	box-shadow: 0 1px 1px rgba(0, 0, 0, 0.2),
	            0 8px 0 -3px #f6f6f6,
	            0 9px 1px -3px rgba(0, 0, 0, 0.2),
	            0 16px 0 -6px #f6f6f6,
	            0 17px 2px -6px rgba(0, 0, 0, 0.2);
}

.todo-count {
	float: left;
	text-align: left;
}

.todo-count strong {
	font-weight: 300;
}

.filters {
	margin: 0;
	padding: 0;
	list-style: none;
	position: absolute;
	right: 0;
	left: 0;
}

.filters li {
	display: inline;
}

.filters li a {
	color: inherit;
	margin: 3px;
	padding: 3px 7px;
	text-decoration: none;
	border: 1px solid transparent;
	border-radius: 3px;
}

.filters li a:hover {
	border-color: rgba(175, 47, 47, 0.1);
}

.filters li a.selected {
	border-color: rgba(175, 47, 47, 0.2);
}

.clear-completed,
html .clear-completed:active {
	float: right;
	position: relative;
	line-height: 20px;
	text-decoration: none;
	cursor: pointer;
}

.clear-completed:hover {
	text-decoration: underline;
}

.info {
	margin: 65px auto 0;
	color: #bfbfbf;
	font-size: 10px;
	text-shadow: 0 1px 0 rgba(255, 255, 255, 0.5);
	text-align: center;
}

.info p {
	line-height: 1;
}

.info a {
	color: inherit;
	text-decoration: none;
	font-weight: 400;
}

.info a:hover {
	text-decoration: underline;
}

/*
	Hack to remove background from Mobile Safari.
	Can't use it globally since it destroys checkboxes in Firefox
*/
@media screen and (-webkit-min-device-pixel-ratio:0) {
	.toggle-all,
	.todo-list li .toggle {
		background: none;
	}

	.todo-list li .toggle {
		height: 40px;
	}

	.toggle-all {
		-webkit-transform: rotate(90deg);
		transform: rotate(90deg);
		-webkit-appearance: none;
		appearance: none;
	}
}

@media (max-width: 430px) {
	.footer {
		height: 50px;
	}

	.filters {
		bottom: 10px;
	}
}
!

JS 

              
                (function (window) {
	'use strict';

	// Your starting point. Enjoy the ride!


	Vue.directive('todo-focus', {
		inserted: function(el){
			el.focus();
		}
	})

	Vue.component('todo-list',{
		template: '#todo-list',
		props: ['todo'],
		data: function(){
			return {
				// Let us set up a isEditing Boolean so that we can know if the user
				// will edit something and we need to change the state to reflect it.
				isEditing: false
			}
		},
		methods: {
			enableEditing: function(){

				this.isEditing = true;
			},

			editTodo: function(todo){
				// todo.todo = todo.todo.trim();
				this.isEditing = false;
			},

			removeTodo: function(todo){
				this.$parent.todos.$remove(todo);
			}
		}
	});

	var app = new Vue({
		el: '#todo-section',
		data: {
			newTodo: '',
			todos: [
			{
				id: 1,
				todo: 'Go to the grocery',
				completed: false
			},
			{
				id: 2,
				todo: 'See the movie',
				completed: true
			},
			{
				id: 3,
				todo: 'Jack Reacher : Tom Cruise',
				completed: false
			}
			]
		},

		methods: {
			addTodo: function(){
				// This will not allow any empty items to be added.
				if(this.newTodo.trim() == ''){
					return;
				}
				this.todos.push({
					todo: this.newTodo.trim(),
					completed: false
				});
				this.newTodo = ''
			}
		}
	});

})(window);
!
999px

Console