CSS performance optimization improves web page loading speed and user experience. The main methods include: 1. Reduce the CSS file size, use compression tools, delete redundant code, merge class names, and avoid global resets; 2. Use critical path CSS, extract and inline the required styles on the first screen, and delay loading of non-critical CSS; 3. Load non-critical CSS asynchronously, use media attributes, dynamic insertion or preload; 4. Optimize selectors and hierarchical structures, give priority to class selectors, avoid long-chain nesting, and control specificity. These methods help speed rendering, reduce resource consumption, while taking into account maintainability.

CSS is an important part of web page loading speed and performance optimization. Many people only focus on the response time of images, scripts or servers, but in fact, the writing, loading method and structural design of CSS will directly affect the rendering speed of pages. Optimizing CSS can not only improve user experience, but also reduce resource consumption.

Reduce CSS file size

The smaller the file size, the faster it loads. Compressing CSS code is the most basic step. Unnecessary spaces, comments, and repeat styles can be automatically removed through the tool. In addition, avoiding deep nesting such as .container .box .item will not only affect readability, but also increase file size.

Some practical suggestions:

• Use CSS compression tools (such as Clean CSS, PostCSS)
• Delete unused CSS styles
• Merge the same or similar class names
• Avoid global resets (such as * { margin: 0; padding: 0 } ), and choose a lighter solution such as normalize.css

Using Critical CSS (Critical CSS)

When the browser loads the page, it first parses HTML and CSS, and then renders the content. If the CSS file is too large or the loading order is unreasonable, the user will see a white screen or the layout will flash. Extracting and inlining the critical path CSS can make the content on the first screen display faster.

Implementation methods include:

• Tools automatically generate key CSS required for the first screen
• Inline this part of CSS into of HTML
• Delay loading of the remaining non-critical CSS

This allows users to see the basic content without waiting for the entire CSS file to load.

Asynchronously loading non-critical CSS

Not all CSS needs to be loaded early in the page loading. For example, styles for printing, styles for hidden components, etc. can be delayed or loaded on demand.

Common strategies are:

• Use media attributes to load specific stylesheets asynchronously (such as media="print" )
• Dynamically insert CSS files (load after judging conditions through JavaScript)
• Use rel="preload" to load certain important but non-critical CSS resources in advance

This will not only ensure the speed of the first screen, but will not miss the subsequent styles.

Optimize selectors and hierarchy

Although modern browsers are already very efficient in handling CSS, inefficient selectors can still cause performance problems. For example, complex selectors like div#main > ul li a will allow the browser to parse from right to left, which is less efficient.

Optimization suggestions include:

• Try to use class selectors ( .class ) instead of ID or tag selectors
• Avoid over-necking and long-chain selectors
• Control CSS specificity to avoid maintenance and performance issues caused by style coverage

In addition, keeping the CSS structure clear also helps with post-maintenance and debugging.

Basically, these common CSS performance optimization methods are all about. It is not necessary to use it all, but the effect will be obvious according to the project situation. The key is to understand the mechanism behind each step, rather than blindly copying it.

The above is the detailed content of Techniques for optimizing CSS performance and load times. For more information, please follow other related articles on the PHP Chinese website!