css 平均分配多列

<style>
    *{margin:0;padding:0;font-size:12px;font-family:'Microsoft YaHei'}
    #nav,#nav2{display:-webkit-box;display:-moz-box;display:box;margin:auto;margin-top:30px;border:10px solid #7f8c8d}
    #nav{width:300px;height:100px;-webkit-box-orient:horizontal;-moz-box-orient:horizontal;box-orient:horizontal}
    #nav2{width:100px;height:300px;-webkit-box-orient:vertical;-moz-box-orient:vertical;box-orient:vertical}
    #nav div,#nav2 div{-webkit-box-flex:1;-moz-box-flex:1;box-flex:1;text-align:center;line-height:100px;font-size:36px;color:#fff}
    #box1{background:#1abc9c}
    #box2{background:#3498db}
    #box3{background:#9b59b6}
    #box4{background:#f39c12}
    #box5{background:#27ae60}
    #box6{background:#c0392b}
</style>
    <!-- 横向排列 -->
    <div id="nav">
        <div id="box1">1</div>
        <div id="box2">2</div>
        <div id="box3">3</div>
    </div>
    <!-- 纵向排列 -->
    <div id="nav2">
        <div id="box4">4</div>
        <div id="box5">5</div>
        <div id="box6">6</div>
    </div>