<button class="btn btn-default" type="button">button标签按钮</button>
<input type="submit" class="btn btn-default" value="input标签按钮"/>
<a href = "##" class="btn btn-default">a标签按钮</a>
<span class="btn btn-default">span标签按钮</span>
<div class="btn btn-default">div标签按钮</div>
因为.btn这个类里面设置了 display: inline-block;