SOURCE

<div class="box">
  <div class="content">内容</div>
  <div class="footer">底部</div>
</div>

.box{
  display: flex;
  flex-direction: column;
  /* justify-content: space-between; */
  align-content: space-between;
}
.box .content{
  flex: 1;
  border: 1px solid #f00;
}