CSS的box-shadow屬性用於在元素周?chē)砑雨幱靶Ч?。其核心用法包括?. 定義水平偏移、垂直偏移、模糊半徑、擴(kuò)展半徑、顏色和inset關(guān)鍵字；2. 使用逗號(hào)分隔添加多個(gè)陰影；3. drop shadows用於讓元素看起來(lái)脫離頁(yè)面，而inset shadows用於內(nèi)部陰影；4. 合理使用以避免影響性能。例如box-shadow: 5px 5px 10px rgba(0, 0, 0, 0.3)創(chuàng)建一個(gè)偏右下角且半透明的陰影。

The box-shadow property in CSS is used to add shadow effects around an element's frame. It can create a subtle depth or dramatic effect depending on how you set the values. The basic idea is that you define one or more shadows — either inset (inside the box) or drop shadows (outside the box).

Basic Syntax and Values

At its core, the box-shadow property takes several values:

• Horizontal offset – how far the shadow moves to the right (positive value) or left (negative).
• Vertical offset – how far the shadow moves down (positive) or up (negative).
• Blur radius (optional) – the higher the number, the more blurred the shadow becomes.
• Spread radius (optional) – how much the shadow expands outward (positive) or contracts inward (negative).
• Color (optional) – sets the color of the shadow.
• Inset keyword (optional) – changes the shadow to be inside the box instead of outside.

Here's a simple example:

 box-shadow: 5px 5px 10px rgba(0, 0, 0, 0.3);

This creates a shadow that's offset to the bottom-right, slightly blurred, and semi-transparent black.

Adding Multiple Shadows

You can apply multiple shadows by separating each shadow definition with a comma. Each shadow will layer on top of the previous one (the first one listed appears on top). This is useful for creating complex lighting effects or layered depth.

For example:

 box-shadow: 
  0 2px 4px rgba(0, 0, 0, 0.1),
  4px 4px 8px rgba(255, 0, 0, 0.3);

In this case, we're stacking two shadows: one soft neutral shadow and another red-toned one at an angle. You can mix and match colors, offsets, blur, and even use inset on some but not others.

A common trick is using a small inset shadow to simulate a pressed or indented button:

 box-shadow: inset 0 1px 2px rgba(0, 0, 0, 0.2);

When to Use Inset vs Drop Shadows

Use drop shadows when you want the element to appear lifted off the page — great for cards, buttons, or floating UI elements.

Use inset shadows when you want a shadow inside the element — often used to give input fields, inner containers, or depressed buttons a sense of depth.

Here are some quick tips:

• A subtle drop shadow like box-shadow: 0 2px 5px rgba(0,0,0,0.1) works well for modern card designs.
• For focus states or active buttons, try inset 0 0 5px rgba(0,0,0,0.2) to create a slight inner glow or depression.
• Don't overdo it — too much blur or spread can make your layout feel cluttered or outdated.

Performance and Browser Support

box-shadow is widely supported across modern browsers. However, heavy use — especially with large blur radii — can affect performance, particularly on mobile devices.

Some things to keep in mind:

• Avoid using extremely large values for blur or spread unless necessary.
• Try to limit the number of shadows per element; more than 3–4 can get expensive.
• Test on actual devices if performance is a concern.

If you're aiming for compatibility with very old browsers (like IE8 and below), remember that those won't support box-shadow at all. But for most modern web projects, it's perfectly safe to use.

基本上就這些。

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