W3School TIY Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
  margin: 0;
  min-width: 250px;
}

/* 包含元素的内边距和边框到总宽度和高度中 */
* {
  box-sizing: border-box;
}

/* 移除列表的边距和内边距 */
ul {
  margin: 0;
  padding: 0;
}

/* 设置列表项的样式 */
ul li {
  cursor: pointer;
  position: relative;
  padding: 12px 8px 12px 40px;
  list-style-type: none;
  background: #eee;
  font-size: 18px;
  transition: 0.2s;
  
  /* 使列表项不可选 */
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

/* 将所有奇数列表项设置为不同的颜色（斑马条纹） */
ul li:nth-child(odd) {
  background: #f9f9f9;
}

/* 鼠标悬停时背景颜色更深 */
ul li:hover {
  background: #ddd;
}

/* 被点击时，添加背景颜色并划去文本 */
ul li.checked {
  background: #888;
  color: #fff;
  text-decoration: line-through;
}

/* 被点击时添加“已选”标记 */
ul li.checked::before {
  content: '';
  position: absolute;
  border-color: #fff;
  border-style: solid;
  border-width: 0 2px 2px 0;
  top: 10px;
  left: 16px;
  transform: rotate(45deg);
  height: 15px;
  width: 7px;
}

/* 设置关闭按钮的样式 */
.close {
  position: absolute;
  right: 0;
  top: 0;
  padding: 12px 16px 12px 16px;
}

.close:hover {
  background-color: #f44336;
  color: white;
}

/* 设置标题样式 */
.header {
  background-color: #f44336;
  padding: 30px 40px;
  color: white;
  text-align: center;
}

/* 清除标题后的浮动 */
.header:after {
  content: "";
  display: table;
  clear: both;
}

/* 设置输入框的样式 */
input {
  margin: 0;
  border: none;
  border-radius: 0;
  width: 75%;
  padding: 10px;
  float: left;
  font-size: 16px;
}

/* 设置“添加”按钮的样式 */
.addBtn {
  padding: 10px;
  width: 25%;
  background: #d9d9d9;
  color: #555;
  float: left;
  text-align: center;
  font-size: 16px;
  cursor: pointer;
  transition: 0.3s;
  border-radius: 0;
}

.addBtn:hover {
  background-color: #bbb;
}
</style>
</head>

<body>

<div id="myDIV" class="header">
  <h2 style="margin:5px">我的待办事项列表</h2>
  <input type="text" id="myInput" placeholder="标题...">
  <span onclick="newElement()" class="addBtn">添加</span>
</div>

<ul id="myUL">
  <li>去健身房锻炼</li>
  <li class="checked">支付账单</li>
  <li>会见比尔盖茨</li>
  <li>去超市购物</li>
  <li>阅读时间</li>
  <li>整理办公室</li>
</ul>

// 点击关闭按钮以隐藏当前列表项
var close = document.getElementsByClassName("close");
var i;
for (i = 0; i < close.length; i++) {
  close[i].onclick = function() {
    var div = this.parentElement;
    div.style.display = "none";
  }
}

// 点击列表项时添加“已选择”符号
var list = document.querySelector('ul');
list.addEventListener('click', function(ev) {
  if (ev.target.tagName === 'LI') {
    ev.target.classList.toggle('checked');
  }
}, false);

// 点击“添加”按钮时创建一个新列表项
function newElement() {
  var li = document.createElement("li");
  var inputValue = document.getElementById("myInput").value;
  var t = document.createTextNode(inputValue);
  li.appendChild(t);
  if (inputValue === '') {
    alert("您必须写一些内容！");
  } else {
    document.getElementById("myUL").appendChild(li);
  }
  document.getElementById("myInput").value = "";

var span = document.createElement("SPAN");
  var txt = document.createTextNode("\u00D7");
  span.className = "close";
  span.appendChild(txt);
  li.appendChild(span);

for (i = 0; i < close.length; i++) {
    close[i].onclick = function() {
      var div = this.parentElement;
      div.style.display = "none";
    }
  }
}
</script>

</body>
</html>