Understand the process of browser parsing HTML

With the development and popularization of Internet technology, each of us uses a browser to browse the web. The browser renders content by parsing HTML documents. Therefore, understanding the process of browsers parsing HTML is very beneficial for a deep understanding of how browsers work and for writing efficient web code.

1. Basic syntax of HTML
Before deeply analyzing the process of browser parsing HTML, first understand the basic syntax of HTML.

HTML consists of a series of tags. A complete HTML page must include elements such as HTML standard declaration, head tag and body tag. Among them, the head tag contains the metadata of the page, such as the page's Title, keywords, description, etc., while the body tag contains the main content of the page.

A complete HTML tag usually contains a start tag and an end tag, for example:

<p>这是一个段落。</p>

where, <p></p> and

is the opening and closing tags of the paragraph, and the content between them is the content of the paragraph.

In addition, there are some tags that are self-closing, that is, there is no closing tag. For example:

<img  src="/static/imghwm/default1.png" data-src="image.jpg" class="lazy" alt="Understand the process of browser parsing HTML" >

The <img alt="Understand the process of browser parsing HTML" > tag is self-closing.

2. The process of browser parsing HTML

The process of browser parsing HTML can be divided into the following steps:

1. Parsing HTML documents

When the browser receives the HTML document, it will first parse it. The HTML parser will parse the HTML document into a tree structure called DOM (Document Object Model), and each HTML tag corresponds to a DOM node.

2. Parsing CSS styles

After parsing the HTML document, the browser also needs to parse the CSS styles to control the style of the page. The browser will parse the CSS style sheet into a style rule and apply it to each node in the DOM tree to make the page display the desired style.

3. Building a rendering tree

After the browser parses the DOM tree and style sheet, it will build and generate a rendering tree based on them. The rendering tree associates elements in the DOM tree, while also taking into account the layout and visual effects in the style sheet. Each node of the rendering tree contains visual information (such as position, size, etc.) for each element.

4. Layout Rendering Tree

After the rendering tree is constructed, the browser will layout the rendering tree, that is, determine the position and size of each element on the page. The browser traverses the render tree and calculates for each element its position and size information on the page.

5. Drawing the rendering tree

The last step is to draw each node in the rendering tree. The browser will draw the content corresponding to each element based on the element's position and size information. The drawing order is usually from background layer to foreground layer.

3. Precautions during the HTML parsing process

When writing HTML code, you also need to pay attention to some issues to avoid affecting the browser's parsing effect on HTML documents.

1. HTML code strictly adheres to the standard

The browser can only correctly parse code that conforms to the HTML standard, so the HTML standard should be strictly adhered to when writing HTML code. Pay attention to the correct use of HTML tags and their attributes, and closure.

2. Avoid nesting errors

Each HTML element has its own semantics and purpose and cannot be abused and nested incorrectly. It is necessary to avoid unclosed elements and too many nesting levels, which will lead to inefficient browser parsing and even directly cause the page to fail to load correctly.

3. Optimize code structure

When writing code, you should also pay attention to optimizing the structure and minimizing the code size and complexity. Proper use of CSS styles, JS scripts and other technologies can improve page loading speed and user experience.

In short, the process of browser parsing HTML is a very complex process, involving the cooperation of multiple links and containing many details. Understanding the process of browsers parsing HTML can help us provide better references for page design and code writing, and can also better understand and apply HTML standards.

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