Optimizing CSS performance can improve loading speed and user experience. Specific methods include: 1. Reduce the complexity of selectors, replace multi-level nesting with class names, and avoid wildcards and excessive pseudo-class combinations; 2. Compress and merge CSS files, and use tools such as CSSNano to delete redundant content; 3. Prioritize loading of critical CSS and delay loading of non-critical styles; 4. Regularly check and delete unused redundant styles, and use DevTools and PurgeCSS to assist in cleaning.

When writing CSS, people often pay more attention to whether the style is correct and whether the layout is beautiful, but few people consider performance and file size first. In fact, optimizing CSS can not only improve page loading speed, but also reduce server pressure, which is also helpful to the user experience. Let’s start from several practical perspectives and talk about how to make CSS lighter and faster without sacrificing functions.

Reduce selector complexity

Many people are accustomed to nesting many layer selectors in CSS, such as structures such as .header .nav .menu li a . Although style control can be achieved, browser parsing such complex selectors need to be matched "from right to left". The more levels, the longer the parsing time.

suggestion:

• Use class names instead of multi-level nesting
• Avoid using wildcard selectors (such as * )
• Try to avoid using property selectors or too many combinations of pseudo-classes

For example, instead of writing:

 .header .nav ul li a {
  color: #333;
}

Why not add a class to the link directly:

 .nav-link {
  color: #333;
}

This not only makes the code clearer, but also makes it easier to maintain.

Compress and merge CSS files

During the development stage, we usually split CSS into multiple files according to modules for easy management. But before going online, it is best to merge these files into one and compress them. Because each additional CSS request will increase HTTP overhead, and the compressed file size can be reduced a lot.

Operation suggestions:

• Use tools such as CSSNano or PostCSS plug-ins to compress
• Merge multiple CSS files into one main stylesheet
• Remove useless comments, spaces, and repeat styles

If you are using build tools, such as Webpack or Vite, you usually have related plug-ins built in, just simple configuration.

Prioritize loading with critical CSS

When rendering the page on the first screen, if the CSS is too large, the user may see blank or unstyled content, which will affect the experience. One solution is to extract the critical CSS (Critical CSS), which is the style that must be used during the first access, first load this part, and the remaining asynchronous load or delay loading.

Common practices:

• Extract critical CSS using tools such as Penthouse
• Dynamically insert non-critical CSS through JavaScript or delay loading in <link rel="stylesheet" media="print">
• Use rel="preload" to load the main CSS resource in advance

This method is especially useful for mobile devices, because the network environment is unstable, and loading faster can improve a lot of experience.

Avoid redundant styles and duplicate rules

During the development process, it is easy to repeatedly define the same class, or write the style but not be used. These redundant contents will make the CSS file larger and larger, but have no practical effect.

Inspection method:

• Use Chrome DevTools' Coverage feature to see which CSSs are not used
• Automatically delete unused styles with tools such as PurgeCSS
• Regularly clean up abandoned component styles in projects

For example, if you find that a button .btn-primary has been defined three times, and the color is different each time, then you must unify the specifications and delete the excess version.

Basically that's it. CSS optimization is not as conspicuous as JS, but it does affect overall performance. Not complicated, but easy to ignore.

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