Summary of page rendering optimization methods in front-end

This time I will bring you a summary of page rendering optimization methods in the front end. What are the precautions for page rendering optimization in the front end? Here are practical cases, let’s take a look.

Cause: network problem, critical rendering path (CRP) problem

Critical Rendering Path (CRP).

1. Describe the entire process from url input to page display?
2. Describe the whole process of html loading?
How to accurately answer questions like the above, then we need to fully understand the browser's critical rendering path. Only by understanding the working principle can we have a better and deeper understanding of the optimization plan for static resources

• Building the DOM tree

构建过程：Bytes->Characters->Tokens->Nodes->Dom

• Build CSSOM tree

构建过程：Bytes->Characters->Tokens->Nodes->CSSOM

##Merge DOM tree and CSSOM tree to build rendering tree

• 1、过滤掉不可见节点(脚本标记、元标记)
  2、过滤掉样式隐藏的节点（display:none）

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Lay out according to the rendering tree and calculate the geometric information of the nodes (layout)
Draw each node on the screen (paint)
First of all, it can be seen from the above five steps that the rendering tree can only be constructed after the DOM tree and CSSOM tree are completed. Therefore, these two steps block the overall rendering. Of course, the DOM tree is It is necessary, it is provided to the page content, and the necessity of CSSOM is not too obvious, so some optimization can be done during the construction process of CSSOM. Before doing optimization, you must first understand these knowledge points.

1、默认情况下，CSS是阻塞渲染的资源
2、我们可以通过媒体查询和媒体类型把一部分CSS标记为不阻塞渲染
（媒体查询的不足就是会严重影响关键渲染路径的性能）
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3、浏览器**会下载所有CSS资源**、无论它阻塞还是不阻塞

Based on the above three knowledge points, you will clearly know that what CSS optimization can do is to perform non-blocking marking according to different CSS usage scenarios and

. If it is necessary CSS, please
load it as early as possible (1. The reference position is higher, 2. Reduce the file size) to the client, thus reducing the need for the first time Rendering blocking2.

The impact on rendering and strategies to reduce the impactFirst, let’s discuss javascript. It can modify all aspects of the web page, content, style, and respond to user interaction. However, javascript can also prevent DOM construction and delay web page rendering. Let's take a look at the dependencies between javascript, DOM, and CSSOM.

javascript can modify content and style
No matter (inline javascript or external javascript file), it will prevent the construction of DOM

• DOM构建过程中如果遇到（非异步加载async）的javascript标签，浏览器将会终止DOM的构建，立即执行javascript。
  这就是为什么非异步执行的javascript要放在尾部或者将可执行代码要放在DOMContentLoaded回调中？
  因为如果该javascript代码操作了未构建完的DOM节点就会因为无法获取该节点而无法执行响应的操作。

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The construction of CSSOM affects the execution of javascript

• 如果在浏览器尚未完成CSSOM的下载和构建时，去运行javascript脚本，那么浏览器会延迟脚本的执行和DOM的构建，直至完成CSSOM的下载和构建。可以这样理解，当出现非异步加载的javascript时，CSSOM构建完成时间是早于javascript的执行，两者早于DOMContentloaded（即DOM构建彻底完成）。

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Not optimized--javascript loads normally

After optimization--javascript asynchronous loading

Based on the above analysis, we can clearly realize that choosing asynchronous loading is the best choice for js that is not necessary to be loaded first.

3. The impact of image on first screen rendering

Images will not prevent the rendering of first screen, but in order to increase the user experience we should consider loading images of appropriate sizes to speed up the rendering of images.

How to evaluate the critical rendering path

The previous content allows us to understand the basic principles of critical path rendering and possible optimization opportunities. Next we need to use some tools to help us evaluate existing CRP performance of the page.

• Testing tool: Lighthouse can quickly test your web page and provide performance reports

• Monitoring tool: Nivigation Timing Api Set up your code, Monitor user performance in real time.

Summary

javascript blocks DOM construction (DOMCommentLoaded trigger is delayed), and the download and completion of css blocks the execution of javascript. In pages without javascript or with only asynchronous javascript, the construction of the DOM and the construction of the CSSOM do not affect each other.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Recommended reading:

Detailed explanation of the steps to use the front-end testing pyramid

How to handle the MySQL database access denial

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