<div class="box">
<div class="box-item _side">1</div>
<div class="box-item _main">2</div>
</div>
.box {
display: flex;
background: #eee;
}
.box-item{
background: red;
height: 50px;
margin:10px;
}
._main{
width: 100%;
flex:1;/*←これ 伸縮率指定しないとIE11でwidth:100%が効かない*/
}
._side{
min-width: 50px;/*←これ*/
width: 50px;/*←これ*/
}
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