Bootstrap官方模板中元素前后为什么都添加了空的before和after伪元素？

Question

在Bootstrap官网的starter template和文档示例里的navbar template查看代码，发现基本所有元素前后都添加了空的before和after伪元素，它们有什么作用吗？
以下是代码截图：

1. starter template的代码

1. navbar template的代码

先在此谢过~

Answer 1

抠出的相关原码，你看看：

*:before,
*:after {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

.clearfix:before,
.clearfix:after,
.dl-horizontal dd:before,
.dl-horizontal dd:after,
.container:before,
.container:after,
.container-fluid:before,
.container-fluid:after,
.row:before,
.row:after,
.form-horizontal .form-group:before,
.form-horizontal .form-group:after,
.btn-toolbar:before,
.btn-toolbar:after,
.btn-group-vertical > .btn-group:before,
.btn-group-vertical > .btn-group:after,
.nav:before,
.nav:after,
.navbar:before,
.navbar:after,
.navbar-header:before,
.navbar-header:after,
.navbar-collapse:before,
.navbar-collapse:after,
.pager:before,
.pager:after,
.panel-body:before,
.panel-body:after,
.modal-footer:before,
.modal-footer:after {
  content: " ";
  display: table;
}

.clearfix:after,
.dl-horizontal dd:after,
.container:after,
.container-fluid:after,
.row:after,
.form-horizontal .form-group:after,
.btn-toolbar:after,
.btn-group-vertical > .btn-group:after,
.nav:after,
.navbar:after,
.navbar-header:after,
.navbar-collapse:after,
.pager:after,
.panel-body:after,
.modal-footer:after {
  clear: both;
}

Answer 2

你看看那些 after 是不是有 clear:both 的属性，有的话就是清除浮动了。