前端优化--dns预获取 dns-prefetch

 不少大型网站使用dns-prefetch 提升页面载入速度。但是同时要注意滥用dns-prefetch，会导致DNS queries have increased to 400 million authoritative queries per month (a high of 120 queries/second) ，dns请求次数每月增加400万。

参考资料：https://www.pinkbike.com/news/DNS-Prefetching-implications.html

                   http://www.tuicool.com/articles/73iYRrv