The core methods for optimizing CSS rendering performance include: 1. Reduce redraw and rearrangement, avoid frequent operation of DOM styles, use transform and opacity to implement animations, and prioritize switching styles through class names; 2. Use will-change and translateZ to enable hardware acceleration, but avoid abuse to save resources; 3. Simplify drawing content and reduce the use of complex styles such as multi-layer shadows and blur filters; 4. Use Chrome DevTools' Performance panel to analyze and locate frequently redraw areas for targeted optimization. These practices can effectively improve page drawing efficiency without affecting the visual effect.

Optimizing CSS rendering performance is actually to reduce the burden on the browser when drawing pages without affecting the visual effect. The core idea is: reduce the redraw area, reduce the drawing complexity, and avoid unnecessary style calculations .

Let’s talk about how to do it from several common perspectives.

Reduce redrawing and rearrangement

When the browser renders a page, if the element style changes, one or more of the three stages of layout, paint, and composite may be triggered. Among them, paint is a relatively resource-consuming part.

To reduce redrawing, you can start from the following points:

• Avoid frequent DOM style operation , such as not modifying the width or background-color of an element multiple times in a loop.
• Animate with transform and opacity attributes, as they usually only trigger the composite phase and do not redraw the entire element.
• Try to use class name switching instead of directly modifying style, so that the style can be updated in batches.

For example: If you want to make a fade animation, using opacity changes is more efficient than switching between display: none and block.

Use will-change and translateZ rationally

These two attributes can tell the browser that "this element may move" and let the browser do some optimizations in advance, such as putting it in a layer alone.

But it should be noted that it cannot be abused . Because each independent layer takes up memory and GPU resources. It is recommended to use it only where high-performance animations are really needed, such as:

 .element {
  will-change: transform, opacity;
}

or:

 .element {
  transform: translateZ(0);
}

These practices can enable hardware acceleration for the browser and improve drawing efficiency.

Simplify drawing content

Some CSS special effects, although good-looking, will significantly increase the drawing cost, such as:

• Multi-layer shadow (box-shadow)
• fuzzy filter (filter: blur())
• Complex gradient background
• Too many border-radius

These will cause the browser to do more calculations when drawing. If you find the page scrolling is stuttering or the animation is not smooth, you can try to simplify these styles.

To give a practical example:
You add three layers of box-shadow to a div, which looks cool, but you have to draw three shadows every time you repaint. If you change it to a single layer or even remove it, the performance will be significantly improved.

Use Chrome DevTools to view drawing

The Performance panel in Chrome Developer Tools can help you see the specific performance of the page during the drawing stage:

• Open the DevTools → Performance tab
• Check "Paint flashing" (sometimes also called "Rendering" setting)
• Refresh the page and observe which areas are frequently redrawn

You can find the "don't move but are moving" parts in this way and then optimize it.

In addition, pay attention to the following points during debugging:

• Are there a lot of complex SVGs or Canvas in the page?
• Are elements frequently redrawn? For example, a background color that is constantly changing?
• Can animation be implemented in a simpler way?

Basically that's it. CSS drawing optimization is not as prone to obvious bottlenecks as JS, but it belongs to the "not difficult to do, but easy to ignore" performance point. As long as you pay attention to writing and select appropriate attributes in daily life, the page can run smoother.

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