CSS implementation principle

CSS is a style sheet language built on markup languages ​​such as HTML, JavaScript, and XML. It is mainly used to describe the style and layout of web pages. Through CSS, you can easily change the fonts, colors, backgrounds, layouts and other styles on the web page to bring a better user experience to the web page. This article mainly introduces the implementation principles of CSS.

1. The basic structure of CSS

CSS styles and HTML tags are separated. In HTML, we define the structure and content of a web page through markup, and CSS is used to specify how to display this content. CSS styles are composed of selectors and declaration blocks. Each selector represents an HTML element, and the declaration block specifies the rules to be applied to this element.

Here is an example of a simple CSS style:

<code>p {
    font-size: 20px;
    color: black;
}</code>

In this example, p is the selector, referring to the paragraph mark in the HTML document <p></p>, within the curly braces is the attribute declaration block, which contains two attributes: font-size and color. These properties describe how this element should be displayed: the font size is 20 pixels and the text is black.

2. How to apply CSS

There are three main ways to apply CSS: inline styles, internal style sheets and external style sheets. The main difference between these applications is where they are applied.

• Inline styles

Inline styles are styles that are applied directly to HTML elements. For example:

<code><p style="color:red;">Hello World!</p></code>

In this example, the style attribute specifies that this paragraph should use a red font color.

• Internal style sheet

The internal style sheet is a style sheet written in the tag, with the following format:

<code>
    <style>
        p {
            color: red;
        }
    </style>
</code>

Define styles through selectors and attribute declaration blocks.

• External style sheet

The external style sheet is written in a separate CSS file and introduced into the HTML document through the <link> tag middle. For example:

<code>
    <link rel="stylesheet" type="text/css" href="style.css">
</code>

In this example, the href attribute specifies the file path of the external style sheet.

3. CSS parsing process

When the browser loads HTML and CSS, they will be parsed separately. The HTML parser parses the HTML document and builds the document tree (DOM tree), interpreting each tag as a node, while the CSS parser parses the CSS document and builds the CSS Object Model (CSSOM), interpreting each selector as an object.

Next, the parser merges the document tree and CSSOM to create a new rendering tree. In this render tree, each node has its style information, indicating where it is on the screen and how it is displayed. Finally, the browser uses the render tree to draw the HTML page.

4. Cascading and inheritance of CSS

The two important concepts of CSS are cascading and inheritance. Cascading represents how multiple style definitions are applied to the same element to determine which style takes precedence. Inheritance indicates whether an element should inherit the styles of its parent element.

• Cascading

When multiple rules are applied to the same element, CSS uses a set of rules to decide which rule has higher priority. Among them, the following rules are arranged from high to low in priority:

<code>1. 在属性声明中使用的`!important`关键字。
2. 内联样式（例如`style`属性）。
3. 选择器中的ID选择器。
4. 选择器中的类选择器、属性选择器和伪类选择器。
5. 选择器中的元素选择器和伪元素选择器。
6. 通用选择器（*）。
</code>

• Inheritance

The values ​​of some attributes can be inherited from ancestor elements. For example, all text elements inherit the font settings of their parent elements. In CSS, inherited and non-inherited properties and elements have been clearly defined in the specification document.

5. CSS optimization tips

• Use external style sheets

If a style is used by multiple web pages, then use them Putting it in a separate CSS file will improve performance. The browser only needs to load this CSS file once instead of loading the same style multiple times.

• Reduce the complexity of the selector

The more complex the selector, the longer the parser needs to process. Keeping selectors as simple as possible can help reduce CSS file size, thereby improving performance.

• Avoid using !important

Although you can force the application of styles through the !important keyword, it will override other Styles with lower priority. Once it's used, it's difficult to undo it, so it's best to avoid it.

• Compress CSS files

You can use CSS compression tools to compress CSS files into a smaller file size. This not only reduces file download time, but also reduces request size and saves bandwidth resources.

6. Conclusion

CSS is one of the foundations of web development, and understanding how it works is very important for learning and using CSS. This article mainly introduces the basic structure, application methods, parsing process, cascading and inheritance, and CSS optimization techniques of CSS. I hope it will be helpful to beginners or developers who want to learn CSS in depth.

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