使用margin负值实现压住相邻边距

 1 <div style=" 1200px;height:900px;margin: 0 auto;border: 1px solid #22DD22;">
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
    <div class="margin"></div>
</div>

.margin {
    position: relative;       //使用定位是为了使用z-index，提高层级         
     200px;
    height: 400px;                    
    border: 1px solid #000000;
    margin-left: -1px;       //边框覆盖，只显示一个边框
    margin-top: -1px;
    float: left;
}
.margin:hover {
    border-color: #22DD22;
    z-index: 99;
}

效果：