浏览器的渲染过程 (Browser’s Rendering Process)

浏览器的渲染过程

The browser’s rendering process is a complex sequence of steps that the browser follows to convert HTML, CSS, and JavaScript into a visually rendered web page on the user’s screen. Understanding this process is crucial for web developers who want to optimize page loading times and ensure smooth, responsive user interfaces.

浏览器的渲染过程是浏览器将 HTML、CSS 和 JavaScript 转换为用户屏幕上可视化的网页所遵循的一系列复杂步骤。理解这一过程对于希望优化页面加载时间并确保流畅、响应式用户界面的 Web 开发人员至关重要。

1. Overview of the Rendering Process

渲染过程概述

1. Receiving the HTML Document

接收 HTML 文档

When the browser receives an HTML document from the server, it begins the process of rendering the page. The rendering process starts as soon as the first chunk of the HTML document is received, which allows the browser to begin parsing and rendering the page before the entire document has finished downloading.

当浏览器从服务器接收到 HTML 文档时，它就开始了渲染页面的过程。渲染过程在接收到 HTML 文档的第一个部分后立即开始，这使得浏览器能够在整个文档下载完成之前开始解析和渲染页面。

2. Parsing the HTML

解析 HTML

The rendering main thread begins by parsing the HTML document, converting it into a Document Object Model (DOM) tree. Each HTML element, such as <div>, <p>, or <h1>, becomes a node in this tree. The DOM tree represents the structure and content of the page, and it is used by the browser to determine how the page should be displayed.

渲染主线程首先解析 HTML 文档，将其转换为文档对象模型 (DOM) 树。每个 HTML 元素，如 <div>、<p> 或 <h1>，都会成为这棵树中的一个节点。DOM 树表示页面的结构和内容，浏览器使用它来确定页面应该如何显示。

3. Pre-parsing CSS and JavaScript

预解析 CSS 和 JavaScript

While the HTML is being parsed, the browser also pre-parses any linked CSS and JavaScript files. This involves fetching these resources and preparing them for execution or application. The CSS is parsed to create the CSS Object Model (CSSOM) tree, which is combined with the DOM tree to determine the styles that should be applied to each element.

在解析 HTML 的同时，浏览器还预解析任何链接的 CSS 和 JavaScript 文件。这包括获取这些资源并为其执行或应用做好准备。CSS 被解析以创建 CSS 对象模型 (CSSOM) 树，并与 DOM 树结合，决定应该应用于每个元素的样式。

4. Constructing the DOM and CSSOM Trees

构建 DOM 和 CSSOM 树

As the HTML and CSS are parsed, the browser constructs two trees:

• DOM Tree: Represents the structure and content of the HTML document.
• CSSOM Tree: Represents the styles applied to the elements in the DOM tree.

The combination of these two trees allows the browser to understand both the structure and the visual presentation of the page.

随着 HTML 和 CSS 的解析，浏览器构建了两棵树：

• DOM 树：表示 HTML 文档的结构和内容。
• CSSOM 树：表示应用于 DOM 树中元素的样式。

这两棵树的结合使得浏览器能够理解页面的结构和视觉呈现。

5. Layout (Reflow)

布局（重排）

Once the DOM and CSSOM trees are constructed, the browser enters the layout phase (also known as reflow). During layout, the browser calculates the size and position of each element on the page based on the information in the DOM and CSSOM trees. This process involves determining how elements fit together, how text flows within containers, and how elements are positioned relative to each other.

一旦 DOM 和 CSSOM 树构建完成，浏览器就进入了布局阶段（也称为重排）。在布局期间，浏览器根据 DOM 和 CSSOM 树中的信息计算页面上每个元素的大小和位置。这个过程包括确定元素如何组合在一起、文本如何在容器中流动以及元素之间的相对位置。

6. Paint

绘制

After the layout is complete, the browser moves to the paint phase. In this phase, the browser fills in pixels for each element based on the layout information and the computed styles. This involves rendering text, images, colors, borders, shadows, and other visual aspects of the elements.

布局完成后，浏览器进入绘制阶段。在这个阶段，浏览器根据布局信息和计算样式填充每个元素的像素。这包括渲染文本、图像、颜色、边框、阴影以及元素的其他视觉方面。

7. Compositing

合成

The final phase in the rendering process is compositing. In this phase, the browser combines the painted layers to create the final image that is displayed on the screen. Each layer may represent a different part of the page, such as a background, a foreground element, or a floating element like a dialog box. These layers are composited together to form the complete visual representation of the page.

渲染过程的最后一个阶段是合成。在这个阶段，浏览器将已绘制的图层组合在一起，以创建最终显示在屏幕上的图像。每个图层可能表示页面的不同部分，如背景、前景元素或浮动元素（如对话框）。这些图层被组合在一起，形成页面的完整视觉呈现。

2. Performance Considerations

性能考虑

1. Critical Rendering Path

关键渲染路径

The critical rendering path refers to the sequence of steps the browser must take to render the initial view of a web page. Optimizing this path is essential for improving page load times. This can involve minimizing the size of CSS and JavaScript files, reducing the number of requests, and deferring non-critical resources.

关键渲染路径是指浏览器必须采取的渲染网页初始视图的一系列步骤。优化这个路径对于提高页面加载时间至关重要。这可能包括最小化 CSS 和 JavaScript 文件的大小、减少请求数量以及推迟非关键资源的加载。

2. Minimizing Reflows and Repaints

最小化重排和重绘

Reflows (or layout changes) and repaints (or visual changes) are costly operations that can slow down the rendering process. Developers should aim to minimize these by avoiding unnecessary DOM manipulations, reducing the complexity of CSS rules, and using CSS properties like transform and opacity to create animations instead of properties that trigger reflows, like width and height.

重排（或布局更改）和重绘（或视觉更改）是代价高昂的操作，可能会减慢渲染过程。开发人员应通过避免不必要的 DOM 操作、减少 CSS 规则的复杂性以及使用 transform 和 opacity 等 CSS 属性来创建动画，而不是触发重排的属性（如 width 和 height），来尽量减少这些操作。

3. Lazy Loading

懒加载

Lazy loading is a technique that delays the loading of non-critical resources (such as images or videos) until they are needed. This helps reduce the initial load time and improves the performance of the critical rendering path.

懒加载是一种延迟非关键资源（如图像或视频）加载的技术，直到需要它们为止。这有助于减少初始加载时间，并提高关键渲染路径的性能。

3. Conclusion

结论

The browser’s rendering process is a critical part of how web pages are displayed to users. From parsing HTML and CSS to building the DOM and CSSOM trees, calculating layouts, and finally painting and compositing layers, each step is essential for creating the final visual output on the screen. Understanding this process allows developers to optimize their code and ensure that their web pages load quickly and perform efficiently, providing users with a smooth and responsive experience.

浏览器的渲染过程是网页如何呈现给用户的关键部分。从解析 HTML 和 CSS 到构建 DOM 和 CSSOM 树，计算布局，最后绘制和合成图层，每一步对于创建屏幕上的最终视觉输出都是至关重要的。理解这个过程使开发人员能够优化他们的代码，并确保他们的网页加载快速且高效运行，为用户提供流畅且响应迅速的体验。