Hints to the browser that might prime the pump for resources you will need.
Preload is the only exception here, being more of an instruction than just a hint. AddyOsmani @ https://plus.google.com/+AddyOsmani/posts/7JvGGPAAuCT

浏览器允我们在网页的 head 中添加特定的 link 标签，提示浏览器进行资源预处理，如资源预加载、域名预解析等等。

• DNS 预解析（Preresolve DNS hostnames for assets）

    <link rel="dns-prefetch" href="http://my-site.com">
  

• 预连接（Begin a connection handshake in the background）

    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
  

• 脚本预加载（Declaratively fetch a resource without executing it）

    <link rel="preload" href="late-script.js" as="script">
  

• 图片预加载（Prefetch a resource for a future navigation）

    <link rel="prefetch" href="/images/large.jpg">
  

• 预渲染（Prerender a page in the background for future nav）

    <link rel="prerender" href="next.html">