SOURCE

<div class="father">
    <div class="left">左边</div>
    <div class="right">右边</div>
</div>

/***第一种
.father {
  display: flex;
  height: 200px;
}
.left {
 width:200px;
}
.right {
 flex: 1;
 width: calc(100% - 200px);
}
*/
/***第二种
.father {
  display: flex;
  height: 200px;
}
.left {
 width:200px;
}
.right {
 width: calc(100% - 200px);
}
*/
.father {
  position: relative;
  height: 200px;
}
.left {
  position: absolute;
  width: 200px;
  height: 100%;
}
.right {
  width: calc(100% - 200px);
  height: 100%;
  margin-left: 200px;
}