<div class="pair odd"></div>
    <div class="pair odd"></div>

    <div class="container" style="overflow: hidden">
        <div class="pair even">开启 BFC</div>
    </div>
    <div class="pair even"></div>

    <div class="pair odd" style="display: inline-block" >开启 BFC</div>
    <div class="pair odd" ></div>
        .pair {
            width: 50px;
            height: 50px;
            /* 上下均有25px, 但实际上两个 item 之间的 margin 是重叠的 */
            margin: 25px;
        }

        .odd {
            background-color: red;
        }

        .even {
            background-color: green;
        }

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