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在web页面中敲定一个高度,以高度得到一条首屏线,在首屏线上方的内容是需要第一之间展示给用户的,它能不能及时的展示是能否留住用户的一个非常重要的因素。研究认为,用户最满意的网页打开速度在2s以内。

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首屏时间

在web页面中敲定一个高度,以高度得到一条首屏线,在首屏线上方的内容是需要第一之间展示给用户的,它能不能及时的展示是能否留住用户的一个非常重要的因素。研究认为,用户最满意的网页打开速度在2s以内。

  • 首屏时间:指用户在没有滚动页面时,看到的内容渲染完成以及可以交互的时间。
  • 加载时间:整个页面滚动到底部,可以渲染并且交互的时间。

在用户访问一个站点时,浏览器会发送一个请求,请求该html文档,当浏览器接收到文档后,就会开始解析并渲染到窗口上。

在这个过程中,有两个非常重要的时间点:

  • Time Of First Byte(首字节时间)
  • Start Render Time(开始渲染时间)

以上两个时间点,将首屏时间分为了如下图三短时间:

Time Of First Byte

首字节时间越长,浏览器就越晚解析文档,首屏时间就越长。

在 HTML5 performance API 中提供了这个时间点。

如果首字节时间越长,那么瓶颈应该是存在于网络、后端程序等方面。

Start Render Time

Start Render Time是指用户屏幕刚开始显示某些内容的时候。

浏览器最开始是一片空白的,在绘制时就会产生变化,有可能是文字、背景、logo等。

非可视化的DOM如head标签是不会展示在浏览器窗口中的,这意味着在浏览器开始渲染之前,至少需要解析完head标签中的内容。

在一般情况下,只要文档中的css文件准备就绪就可以开始绘制,而我们的css文件都是写在head标签中的。因此,我们可以认为,只要是开始渲染body标签或者解析完head标签的时刻就是 Start Render Time

根据这一特性,我们可以在body标签开始处或者head标签结束处埋下脚本用来获取 Start Render Time。

减少首屏使用的方法

  • 首屏css内嵌
  • css放 head内,js放
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