html5shiv.js and respond.min.js

html5shiv.js: Solve the problem that browsers below ie9 do not recognize the newly added tags of html5 and cause CSS to not work.

respond.min.js: Let browsers that do not support css3  Media Query, including other browsers such as IE6-IE8, support query.

Since there are still a large number of users in IE6/IE7/IE8, in order to allow website browsers to access HTML5 websites normally, these two solutions are still needed.

The following is the cdn address writing of the two solutions:

<!--[if lt IE 9]>

　　<script src="//cdn.bootcss.com/respond.js/1.4.2/respond.js"></script>

 　　<script src="http://cdn.bootcss.com/html5shiv/3.7.2/html5shiv.min.js"></script>

<![endif]—>

There is also a css3-mediaqueries.js  that allows ie8 and below to support css3 media queries.  It can also be introduced like the above, but there will be a splash screen, which is not particularly recommended. The above is the most commonly used.

Here you can find other CDN addresses of bootstrap http://www.bootcdn.cn/