弹框上下居中，高度100%

<style>
html,body{ 
  margin:0px; 
  height:100%;
} 
.container { 
  height: 100%;
}
.c2{ 
  100%; 
  background:#09F;
  display: flex;
  justify-content: center;
  align-items: center;
}
.alert {
  border: 1px solid #f00;
}
</style>

<div class="container c2" >
    <div class="alert">
      <p>啦啦啦</p>
      <p>啦啦啦</p>
      <p>啦啦啦</p>
      <p>啦啦啦</p>
      <p>啦啦啦</p>
      <p>啦啦啦</p>
    </div>
  </div>

　　弹框上下居中