使用<ul>元素来创作无序列表，列表中的每个项目都被置于起始标签和结束标签之间
<!DOCTYPE html>
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<html>
<head>
<title>TODO supply a title</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
<body>
<div>无序列表</div>
<ul>
<li>一个项目</li>
<li>一个项目</li>
<li>一个项目</li>
<li>一个项目</li>
</ul>
</body>
</html>