<div class="container">

            <h2>实例</h2>

            <div class="pull-left">元素浮动到左侧</div>

            <div class="pull-right">元素浮动到右侧</div>

        </div>

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.container{margin-right:auto;margin-left:auto;padding-left:15px;padding-right:15px;}

.pull-left{float:left !important;}

!important会提升该条规则的优先级，如果有如下2条规则
.pull-left{color:red !important;}
.pull-left{color:green;}
这样pull-left文字的颜色就是红色
如果不加!important就会被第二条的规则覆盖显示为绿色了