margin表示一个元素的外边距。取值为正值时，表示相对于正常流离邻近元素更远，而取负值时，使其更近
但是，设置margin后，四个方向的表现形式不同
自身发生移动：top、left
<div id="negtive_top">
<div>margin-top: -50px;自身向上移动50px</div>
</div>
CSS
#negtive_top{
background-color: green;
border: 2px solid red;
}
#negtive_top div{
background-color: yellow;
border: 2px solid red;
opacity: 0.5;
margin-top: -50px;
}
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<div id="negtive_left">
<div>margin-left: -50px;自身向左移动50px</div>
</div>
CSS
#negtive_left{
background-color: green;
border: 2px solid red;
}
#negtive_left div{
background-color: yellow;
opacity: 0.5;
margin-left: -50px;
border: 2px solid red;
}
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邻近元素移动：right、bottom
<div id="negtive_right">margin-right: -50px;右边的元素向左移动50px</div>
<div></div>
CSS
#negtive_right{
background-color: green;
border: 2px solid red;
float: left;
margin-right: -50px;
}
#negtive_right+div{
background-color: yellow;
opacity: 0.5;
border: 2px solid red;
float: left;
}
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<div id="negtive_bottom">margin-bottom: -50px;下方的元素向上移动50px</div>
<div></div>
CSS
#negtive_bottom{
background-color: green;
border: 2px solid red;
margin-bottom: -50px;
}
#negtive_bottom +div{
background-color: yellow;
opacity: 0.5;
border: 2px solid red;
}
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参考
https://developer.mozilla.org/zh-CN/docs/Web/CSS/margin
