css 关于空白叠加

css代码

body{
margin: 0;
}
.do{
background-color: red;
overflow: hidden;
zoom: 1;
}
.dm{
margin-top: 10px;
background-color: green;
overflow: hidden;
zoom: 1;
}
.di{
margin-top: 5px;
background-color: blue;
}