等高布局 - table-cell

HTML

<div class="box">
    <div class="side"></div>
    <div class="main"></div>
</div>

CSS

.box {
    display: table;
    width: 100%;
}

.side,
.main {
    display: table-cell;
}

.side {
    height: 120px;
    background: #515151;
    width: 200px;
}

.main {
    background-color: #919191;
    height: 180px;
}