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CSS

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HTML 

              
                <div class="container">
	<button type="buton" id="add" title="Add an item">+</button>
	<h1 id="title">My Todo List</h1>

	<ul class="list" aria-describedby="title">
		<li>
			<input type="checkbox" name="todolist" id="item-1" aria-labelledby="item-label-1">
			<label for="item-1"></label>
			<span id="item-label-1" contenteditable="true">Your first task</span>
		</li>
		<li>
			<input type="checkbox" name="todolist" id="item-2" aria-labelledby="item-label-2">
			<label for="item-2"></label>
			<span id="item-label-2" contenteditable="true"></span>
		</li>
	</ul>

	<button type="button" id="reset">Reset the list</button>
</div>
!

CSS 

              
                :root {
	--primary: #3d7e9a;
}

body {
	width: 370px;
	max-width: 100%;
	margin: 0 auto;
	font-family: Helvetica, Arial, sans-serif;
	background: #f5f9fa;
	color: #444;
}

h1 {
	margin-top: -40px;
	display: flex;
	align-items: center;
	color: var(--primary);
}

.container {
	margin: 40px 16px;
	padding: 32px 24px;
	background: white;
	box-shadow: 0 16px 16px -8px rgba(0, 0, 0, .1);
	border: 1px solid rgba(0,0,0,.025);
}

/*
 * Add button
 */
[id="add"] {
	position: sticky;
	top: 40px;
	width: 48px;
	height: 48px;
	margin-left: 100%;
	padding-bottom: 4px;
	border: 0;
	font-size: 1.5em;
	line-height: 48px;
	color: white;
	background: var(--primary);
	border-radius: 50%;
	transition: all .275s;
	cursor: pointer;
}

[id="add"]:hover,
[id="add"]:focus {
	background: #22586f;
	transform: translateY(-2px);
	box-shadow: 0 4px 8px -3px rgba(0,0,0,.6);
}

[id="add"]:active {
	transform: translateY(-1px);
	box-shadow: 0 2px 4px -3px rgba(0,0,0,.8);
}

/**
 * Reset button
 */
[id="reset"] {
	margin-top: 32px;
	background: none;
	padding: 0;
	border: 0;
	font-size: .875em;
	font-weight: bold;
	color: darksalmon;
	cursor: pointer;
	text-decoration: underline;
}

/**
 * THE LIST
 */
.list {
	display: flex;
	flex-direction: column;
	margin: 0;
	padding: 0;
	list-style: none;
}

.list li {
	position: relative;
	margin: 4px 0;
	padding: 13px 16px 5px;
	border-top: 1px solid rgba(0,0,0,.1);
	line-height: 1.55;
	background: white;
}

.list li.arriving {
	animation: arriving .3s ease-in-out 1 forwards;
}

.list .done {
	padding: 4px 40px;
	margin: 0 -24px;
	border: 0;
	order: 100;
	background: #f5f9fa;
}

.list .done label::before,
.list .done label::after,
.list .done span {
	opacity: .5!important;
}

.list .first-done {
	margin-top: 40px;
}

.list .first-done:before {
	content: 'Things done';
	display: block;
	padding: 8px 0 16px;
	font-weight: bold;
	color: var(--primary);
}

.list span {
	padding: 5px 8px 3px;
	margin-top: -.1em;
	display: inline-block;
	vertical-align: top;
	width: calc(100% - 56px);
	min-height: 1.5em;
	transition: all .275s;
	border-radius: 4px;
}

.list span:empty {
	background: #EEE;
}

.list span:empty::before {
	content: 'Write something';
	opacity: .4;
}

.list span:focus {
	background: var(--primary);
	outline: none;
	color: #FFF;
}

/**
 * Checkbox styling
 * with feature support selector
 * @see: (EN) https://www.creativejuiz.fr/blog/en/tutorials/customize-checkbox-and-radio-button-with-css - (FR) 
 */
[type="checkbox"]:not(:checked), 
[type="checkbox"]:checked {
	position: absolute;
	left: 0;
	opacity: 0.01;
}

/* Preparing the label */
[type="checkbox"]:not(:checked) + label,
[type="checkbox"]:checked + label {
	position: relative;
	padding-left: 1.75em;
	font-size: 1.05em;
	line-height: 1.7;
	cursor: pointer; 
}

/* Box aspect */
[type="checkbox"]:not(:checked) + label:before,
[type="checkbox"]:checked + label:before {
	content: '';
	position: absolute;
	left: 0;
	top: -.25em;
	width: 1.4em;
	height: 1.4em;
	border: 2px solid #aaa;
	background: #FFF;
	border-radius: .2em;
	box-shadow: inset 0 1px 3px rgba(0,0,0, .1), 0 0 0 rgba(203, 34, 237, .2);
	transition: all .275s;
}

/* Check aspect */
[type="checkbox"]:not(:checked) + label:after,
[type="checkbox"]:checked + label:after {
	content: '✕';
	position: absolute;
	top: .475em;
	left: .19em;
	font-size: 1.4em;
	color: var(--primary);
	line-height: 0;
	transition: all .2s;
}

/* Unchecked aspect */
[type="checkbox"]:not(:checked) + label:after {
	opacity: 0;
	transform: scale(0) rotate(45deg);
}

/* Checked aspect */
[type="checkbox"]:checked + label:after {
	opacity: 1;
	transform: scale(1) rotate(0);
}

/* Accessibility focus */
[type="checkbox"]:checked:focus + label:before,
[type="checkbox"]:not(:checked):focus + label:before {
	box-shadow: inset 0 1px 3px rgba(0,0,0, .1), 0 0 0 6px rgba(34, 170, 247, .2);
}

@keyframes arriving {
	0% {
		transform: translateX(-20px);
		opacity: 0;
	}
	100% {
		transform: translateX(0);
		opacity: 1;
	}
}
!

JS 

              
                /**
 * CodePen is buggy and doesn't get the LocalStorage data at the first load of JS :/
 */
const add   = document.getElementById('add'),
		reset = document.getElementById('reset');

var saveListData = function(event){
	if ( window.localStorage ) {
		var lis  = document.querySelectorAll('.list li'),
			data = {},
			i    = 0;

		lis.forEach(function(el){
			var item = {};
			item['status'] = el.querySelector('input').checked;
			item['content'] = el.querySelector('span').innerHTML;
			data[i] = item;
			i++;
		});

		localStorage.setItem('my-awesome-todolist', JSON.stringify(data) );
	}
}

/**
 * Add an Item into the list
 */
add.addEventListener('click', function(){
	var last = document.querySelector('.list li:last-child'),
		 num = last.querySelector('input').id.split('-')[1];

	var clone = last.cloneNode(true);

	num = parseInt(num) + 1;
	clone.querySelector('input').setAttribute('id', 'item-' + num );
	clone.querySelector('input').setAttribute('aria-labelledby', 'item-label-' + num );
	clone.querySelector('input').removeAttribute('checked');
	clone.querySelector('input').checked = false;

	clone.querySelector('label').setAttribute('for', 'item-' + num );
	clone.querySelector('span').setAttribute('id', 'item-label-' + num );
	clone.querySelector('span').innerHTML = '';

	clone.className = '';

	last.insertAdjacentElement('afterend', clone);

	document.getElementById( 'item-' + num ).scrollIntoView({
		behavior: 'smooth'
	});
	
	document.getElementById( 'item-' + num ).closest('li').classList.add('arriving');

	document.getElementById( 'item-label-' + num ).focus();
});

/**
 * Reset the list
 */
reset.addEventListener('click', function(){
	var clone = document.querySelector('.list li:last-child').cloneNode(true);

	clone.className = '';
	clone.querySelector('span').innerHTML = '';
	clone.querySelector('input').removeAttribute('checked');

	document.querySelector('.list').innerHTML = '';
	document.querySelector('.list').appendChild( clone );

	saveListData();
});

/**
 * Save the data into localStorage
 */
if ( window.localStorage ) {
	var changeItemStatus = function(event){
		var parent = event.target.parentNode,
			lis    = document.querySelectorAll('.list li'),
			fdone  = document.querySelector('.first-done');

		if ( fdone ) {
			fdone.classList.remove('first-done');
		}

		if ( parent.classList.contains('done') ) {
			parent.classList.remove('done');
		} else {
			parent.classList.add('done');
		}

		for ( let el of document.querySelectorAll('.done') ) {
			el.classList.add('first-done');
			break;
		};
	},
	firstDone = false;

	// If data exists, build the list
	if ( localStorage.getItem('my-awesome-todolist') ) {

		var data = JSON.parse( localStorage.getItem('my-awesome-todolist') ),
			 html = '';

		Object.keys(data).forEach(function(k){

			if ( data[k].status === true && firstDone ==! 'already' ) {
				firstDone = true;
			}

			console.log(firstDone);

			html += '<li' + ( data[k].status ? ' class="done' + ( firstDone === true ? ' first-done' : '' ) + '"' : '' ) + '><input type="checkbox" name="todolist" id="item-' + k + '" aria-labelledby="item-label-' + k + '"' + ( data[k].status ? ' checked="checked"' : '' ) + '><label for="item-' + k + '"></label><span id="item-label-' + k + '" contenteditable="true">' + data[k].content + '</span></li>';

			if ( firstDone !== 'already' ) {
				firstDone = firstDone === true ? 'already' : false;
			}
		});

		// Write the existing list.
		document.querySelector('.list').innerHTML = html;
	}

	// Delegation of blur and change events
	document.querySelector('.list').addEventListener('blur', saveListData, true);
	document.querySelector('.list').addEventListener('change', saveListData, true);

	// Delegation of change for ordering items
	document.querySelector('.list').addEventListener('change', changeItemStatus, true);
}
!
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