bootstrap - 版式
col-lg-3/col-md-3/col-sm-3/col-lg-3:它们各自有自己的适应范围,一般而言,需要把它们都写上;他们的的父元素必须设置“row”class样式;
col-**-push-3:把元素块向右推3格;col-**-pull-3:把元素块向左推3格
<meta name="viewport" content="width=device-width, initial-scale=1">:移动端优先
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<script src="https://cdn.bootcss.com/jquery/1.12.4/jquery.min.js"></script>
<script src="js/bootstrap.min.js"></script>:上下顺序不能变