<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>网页标题</title>
<style>
body{
margin: 0px;
padding: 0px;
}
.uni{
width:200px;
height:200px;
/*浮动：方向*/
float: left;
}
#first{
background:yellow;
margin-right: 10px;
margin-bottom: 10px;
}
#second{
background:red;
margin-bottom: 10px;
}
#third{
background:red;
margin-right: 10px;
}
#fourth{
background:yellow;
}
div{}
</style>
</head>
<body>
<div >
<div id="first" class="uni">

</div>
<div id="second" class="uni">
</div>

<div id="third" class="uni">
</div>
<div id="fourth" class="uni">
</div>
</div>
</body>
</html>