box-flex

<ul id="box">
    <li>a</li>
    <li>b</li>
    <li>c</li>
</ul>#box{display:box;width:240px;height:100px;margin:0;padding:10px;list-style:none;}

#box li:nth-child(1){box-flex:1;}
#box li:nth-child(2){box-flex:1;}
#box li:nth-child(3){box-flex:2;}

来源：https://www.cnblogs.com/jjrr/p/11375105.html