理解浏览器如何解析HTML和渲染内容

Understanding How Browsers Parse HTML and Render Content

The process by which a browser parses HTML and renders content is a critical part of how web pages are displayed to users. This process involves several steps, starting from the moment the browser receives an HTML document from the server, to the final stage where the web page is fully rendered on the screen. Understanding this process is essential for optimizing web performance and ensuring that web pages load quickly and efficiently.

浏览器解析 HTML 并渲染内容的过程是网页如何呈现给用户的关键部分。这个过程包括从浏览器接收到来自服务器的 HTML 文档的那一刻开始，到最终在屏幕上完全渲染网页的各个步骤。理解这个过程对于优化网页性能和确保网页快速高效加载至关重要。

1. The Initial Request and Response

初始请求和响应

1.1. Making the HTTP Request

发起 HTTP 请求

When a user enters a URL into the browser’s address bar, the browser sends an HTTP request to the server hosting the website. This request typically includes information about the desired resource (the HTML file), the browser type, and any relevant cookies or authentication tokens.

当用户在浏览器的地址栏中输入 URL 时，浏览器会向托管该网站的服务器发送 HTTP 请求。该请求通常包括关于所需资源（HTML 文件）、浏览器类型以及任何相关的 Cookie 或身份验证令牌的信息。

1.2. Receiving the HTML Document

接收 HTML 文档

The server responds by sending back the requested HTML document. The browser begins processing this document as soon as it starts receiving it, rather than waiting for the entire file to be downloaded. This allows the browser to start rendering the page as quickly as possible.

服务器通过发送请求的 HTML 文档进行响应。浏览器一接收到该文档就会开始处理它，而不是等待整个文件下载完成。这使得浏览器能够尽快开始渲染页面。

2. Parsing the HTML Document

解析 HTML 文档

2.1. Tokenization

词法分析

The first step in parsing the HTML document is tokenization. The browser’s HTML parser reads the document character by character and breaks it down into tokens. Tokens are the basic building blocks of the document and include tags (e.g., <div>), attributes (e.g., class="container"), and text content.

解析 HTML 文档的第一步是词法分析。浏览器的 HTML 解析器逐字读取文档并将其分解为标记。标记是文档的基本构建块，包括标签（例如 <div>）、属性（例如 class="container"）和文本内容。

2.2. Building the DOM Tree

构建 DOM 树

As the browser tokenizes the HTML, it begins to construct the Document Object Model (DOM) tree. The DOM tree is a hierarchical representation of the HTML document, where each node in the tree corresponds to an element or a piece of content in the document. The root of this tree is the <html> element, with child nodes representing the <head>, <body>, and other elements within the document.

随着浏览器对 HTML 进行词法分析，它开始构建文档对象模型 (DOM) 树。DOM 树是 HTML 文档的层次表示，其中树中的每个节点对应于文档中的一个元素或一部分内容。该树的根是 <html> 元素，子节点表示文档中的 <head>、<body> 和其他元素。

<!DOCTYPE html>
<html>
<head>
    <title>Sample Page</title>
</head>
<body>
    <h1>Hello, World!</h1>
    <p>This is a sample page.</p>
</body>
</html>

Example:
In this HTML document, the browser creates a DOM tree with <html> as the root node, <head> and <body> as child nodes, and further nodes representing the <title>, <h1>, and <p> elements.

示例：
在这个 HTML 文档中，浏览器创建了一个以 <html> 为根节点、以 <head> 和 <body> 为子节点的 DOM 树，并进一步创建代表 <title>、<h1> 和 <p> 元素的节点。

2.3. Handling Scripts

处理脚本

When the browser encounters a <script> tag, it typically pauses the construction of the DOM tree to execute the JavaScript code. This is because JavaScript can modify the DOM and CSSOM, so the browser needs to ensure the script’s effects are applied before continuing with the parsing process. Scripts can either block rendering (synchronous scripts) or be loaded asynchronously to prevent blocking (using the async or defer attributes).

当浏览器遇到 <script> 标签时，它通常会暂停 DOM 树的构建，以执行 JavaScript 代码。这是因为 JavaScript 可以修改 DOM 和 CSSOM，因此浏览器需要确保脚本的效果在继续解析过程之前被应用。脚本可以阻止渲染（同步脚本），也可以异步加载以防止阻塞（使用 async 或 defer 属性）。

<script>
    console.log('Script executed!');
</script>

Example:
In this example, the browser pauses DOM construction to execute the script, which logs a message to the console.

示例：
在此示例中，浏览器暂停 DOM 构建以执行脚本，该脚本会在控制台打印消息。

3. Building the CSSOM Tree

构建 CSSOM 树

3.1. Parsing CSS

解析 CSS

While the HTML is being parsed, the browser also fetches and parses any linked CSS files or inline styles. The CSS parser reads the CSS rules and creates a CSS Object Model (CSSOM) tree. The CSSOM tree represents the styles that need to be applied to each element in the DOM tree.

在解析 HTML 的同时，浏览器还会获取并解析任何链接的 CSS 文件或内联样式。CSS 解析器读取 CSS 规则并创建一个 CSS 对象模型 (CSSOM) 树。CSSOM 树表示需要应用于 DOM 树中每个元素的样式。

body {
    font-family: Arial, sans-serif;
}

h1 {
    color: blue;
}

Example:
For this CSS, the browser creates a CSSOM tree that applies the specified font to the <body> element and sets the color of <h1> elements to blue.

示例：
对于此 CSS，浏览器创建了一个 CSSOM 树，将指定的字体应用于 <body> 元素，并将 <h1> 元素的颜色设置为蓝色。

3.2. Combining the DOM and CSSOM

结合 DOM 和 CSSOM

Once the DOM and CSSOM trees are complete, the browser combines them to create a render tree. The render tree includes only the elements that need to be displayed on the screen, along with their computed styles.

一旦 DOM 和 CSSOM 树完成，浏览器将它们结合起来创建一个渲染树。渲染树仅包含需要显示在屏幕上的元素及其计算样式。

4. Layout and Painting

布局和绘制

4.1. Layout (Reflow)

布局（重排）

The layout phase, also known as reflow, involves calculating the position and size of each element based on the render tree. The browser determines the exact coordinates where each element will appear on the screen.

布局阶段，也称为重排，涉及根据渲染树计算每个元素的位置和大小。浏览器确定每个元素在屏幕上出现的确切坐标。

/* Example Layout Properties */
body {
    margin: 0;
}

h1 {
    margin-top: 20px;
}

Example:
In this example, the browser calculates the layout, determining that the <h1> element should be positioned 20 pixels from the top of the <body> element.

示例：
在这个示例中，浏览器计算布局，确定 <h1> 元素应位于 <body> 元素顶部 20 像素处。

4.2. Painting

绘制

After the layout is complete, the browser proceeds to the paint phase, where it fills in the pixels on the screen. This involves drawing the content, including text, colors, images, borders, and shadows, based on the computed styles.

布局完成后，浏览器进入绘制阶段，在屏幕上填充像素。这包括根据计算的样式绘制内容，包括文本、颜色、图像、边框和阴影。

4.3. Compositing

合成

Finally, the browser composites the different layers created during the painting phase. Each layer may represent a different part of the page, such as a background, a floating element, or a foreground image. The browser combines these layers to produce the final image that is displayed on the screen.

最后，浏览器将绘制阶段创建的不同图层进行合成。每个图层可能表示页面的不同部分，如背景、浮动元素或前景图像。浏览器将这些图层组合在一起，生成最终显示

在屏幕上的图像。

5. Performance Optimization Considerations

性能优化考虑

5.1. Minimize Blocking Resources

最小化阻塞资源

To improve performance, it’s important to minimize blocking resources such as large CSS files or synchronous JavaScript. Use techniques like deferred loading (defer or async for scripts) and inline critical CSS to speed up the initial rendering.

为了提高性能，重要的是最小化阻塞资源，如大型 CSS 文件或同步 JavaScript。使用延迟加载技术（脚本的 defer 或 async）和内联关键 CSS 来加快初始渲染。

5.2. Optimize Layout and Paint

优化布局和绘制

Reduce the complexity of your layouts and minimize the number of elements that require reflow and repaint. Use techniques like transform and opacity for animations to avoid triggering costly reflows.

减少布局的复杂性，最小化需要重排和重绘的元素数量。使用 transform 和 opacity 等技术进行动画，以避免触发高成本的重排。

5.3. Lazy Loading

懒加载

Implement lazy loading for images, videos, and other non-critical resources to defer their loading until they are needed. This helps reduce the initial load time and improves the user experience.

为图像、视频和其他非关键资源实现懒加载，以推迟其加载直到需要时。这有助于减少初始加载时间并改善用户体验。

6. Conclusion

结论

The process of parsing HTML and rendering content is a complex sequence of steps that browsers perform to display web pages. By understanding how this process works, developers can optimize their code and design pages that load faster and perform better. Efficient parsing and rendering are key to providing users with a smooth, responsive, and enjoyable web experience.

解析 HTML 和渲染内容的过程是浏览器为了显示网页而执行的一系列复杂步骤。通过理解这一过程的工作原理，开发人员可以优化他们的代码，并设计加载更快、性能更好的页面。高效的解析和渲染是为用户提供流畅、响应迅速且愉快的 Web 体验的关键。

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JavaScript 101: Understanding How Browsers Parse HTML and Render Content