1.实现盒子的水平垂直居中

.parent{
width:200px;
height:200px;
display:flex;
align-items: center;
justify-content: center;
border:1px solid black;
}
.child{width:100px;height:100px;background:yellowgreen}
<div class="parent">
<div class="child">
</div>
</div>
2.子元素水平排列一行,均分父元素宽度(如果采用百分比实现,当子元素个数变化时需要修改样式,用flex布局则不需要)

.box{
width:400px;
display:flex;
border:1px solid black;
}
.child{ margin:10px;flex:1;background:yellowgreen}
<div class="box">
<div class="child">1</div>
<div class="child">2</div>
<div class="child">3</div>
</div>
3.左侧固定宽度,右侧宽度随父容器宽度变化而变化布局

.box{
/* width:500px; */
display:flex;
border:1px solid black;
}
.left{
width:100px;
background:blue;
}
.right{
flex:1;
background:yellowgreen;
}
<div class="box">
<div class="left">1</div>
<div class="right">2</div>
</div>