为警告框添加一个可选的 .alert-dismissible 类和一个关闭按钮。

<div class="alert alert-warning alert-dismissible" role="alert">
  <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>
  <strong>Warning!</strong> Better check yourself, you‘re not looking too good.
</div>

可关闭的警告框