ㄧ.alert提示組件

<div class="component demo">
  <div class="alert alert-primary" role="alert">
   <h4>Alert</h4>
   hello~ <a href="#" class="alert-link">see more.</a>
 </div>
</div>

.component.demo{
 padding: 50px;
}

重點：
1.alert定義為alert，alert-primary為樣式，多種樣式都在下面的範例
2.使用 .alert-link 來快速為警報中的連結增添合適的色彩。

二.alert的樣式

<div class="alert alert-primary" role="alert">
  A simple primary alert—check it out!
</div>
<div class="alert alert-secondary" role="alert">
  A simple secondary alert—check it out!
</div>
<div class="alert alert-success" role="alert">
  A simple success alert—check it out!
</div>
<div class="alert alert-danger" role="alert">
  A simple danger alert—check it out!
</div>
<div class="alert alert-warning" role="alert">
  A simple warning alert—check it out!
</div>
<div class="alert alert-info" role="alert">
  A simple info alert—check it out!
</div>
<div class="alert alert-light" role="alert">
  A simple light alert—check it out!
</div>
<div class="alert alert-dark" role="alert">
  A simple dark alert—check it out!
</div>

codepen
參考：bootstrap4