Describe CSS performance optimization techniques
Jul 16, 2025 am 12:52 AM
The key to optimizing CSS performance is to reduce complexity, avoid re-arrangement and repainting, and use selectors and structured management styles. 1. Use simple class selectors to avoid over-necking and complex pseudo-class selectors to improve matching speed; 2. Animation prioritizes transformation and opacity to reduce layout attribute changes to avoid expensive rearrangements; 3. Use class switching when batch-modifying styles or remove elements first and then operate to reduce the performance loss caused by frequent redrawing and rearrangements; 4. Simplify CSS files, merge duplicate rules, delete useless code, and use modular naming specifications such as BEM to improve maintainability.
CSS is a key part of web rendering, but many people ignore its impact on performance. Optimizing CSS can not only speed up page loading, but also improve page rendering efficiency. The key is to reduce complexity, avoid rearrangement and redrawing, and use selectors and structured management styles.
Use selectors reasonably to avoid over-necking
Many people like to write complex CSS selectors like .container > .item:nth-child(odd) , which actually increases the burden of browser parsing. The simpler the selector, the faster the match.
- Class selector (
.class) is recommended because it is the fastest way. - Try to avoid using descendant selectors (such as
div span), especially when the levels are too deep. - Reduce the use of pseudo-classes and attribute selectors, which are less efficient when matching in large numbers of elements.
For example:
/* Not very recommended*/
nav ul li a {
color: #333;
}
/* More efficient*/
.nav-link {
color: #333;
}Avoid frequent re-arrangement and re-painting
When CSS changes layout-related attributes (such as width, height, margin), reflow will be triggered, which is very expensive. If you just change the color or background, it will only trigger repaint, which is relatively less expensive.
- Try to use
transformandopacityfor animations, these two properties will not cause re-arrangement. - When batch modifying the DOM style, you can remove the element first and then reinsert it, or use the CSS class toggle instead of multiple JS operations.
- Using
will-changeortranslateZcan tell the browser in advance which elements may change and prepare for optimization, but don't abuse it.
For example:
/* Recommended animation method*/
.animate {
transition: transform 0.3s ease;
}
.animate:hover {
transform: scale(1.1);
}Simplify and organize your CSS files
As projects grow, CSS files tend to become bloated. Merging duplicate rules, deleting useless styles, and splitting module files can all effectively reduce volume and improve maintenance.
- Use a tool such as PurgeCSS to delete unused CSS.
- Merge multiple CSS files into one main file to reduce HTTP requests.
- Use BEM or similar naming specifications to avoid style conflicts and duplicate definitions.
For example, if you find code like this:
.btn { padding: 10px; }
.primary-btn { padding: 10px; background: blue; }It can be simplified to:
.btn {
padding: 10px;
}
.btn-primary {
background: blue;
}Basically that's it. CSS performance optimization is not as conspicuous as JS, but the impact is tangible. Not complicated but easy to ignore.
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