Header
This is an alert with 36px height.
This is an alert with 24px height.

<div class="alert alert-danger">
    <button type="button" class="close" aria-label="Close">
        <span aria-hidden="true">&times;</span>
    </button>
    <div class="alert-item">
        <span>
            This is an alert with 36px height.
        </span>
    </div>
</div>
<div class="alert alert-success alert-sm">
    <div class="alert-item">
        <span>
            This is an alert with 24px height.
        </span>
    </div>
</div>