px是固定單位，適用於需要精確控制尺寸的場景，如邊框或小圖標(biāo)；rem是相對單位，基於根元素字體大小，適合響應(yīng)式設(shè)計和提升可訪問性。使用px時元素尺寸固定，不受用戶設(shè)置影響；使用rem時，調(diào)整根元素字體大小即可全局縮放。常見做法是用rem設(shè)置字體和佈局間距，用px處理需精準(zhǔn)控制的部分。兩者可混合使用，但應(yīng)測試用戶修改瀏覽器默認(rèn)字體大小時的佈局表現(xiàn)。

rem and px are both units used in CSS for sizing elements, but they behave differently depending on context — especially when it comes to scalability and user preferences.

What px really means

px stands for "pixels," and in CSS, it's not exactly the same as your screen's physical pixels. It's what's called a "reference pixel" — a fixed unit that doesn't change based on user settings or parent element sizes.

• Most browsers default to displaying 16px as the base font size.
• If you set an element to font-size: 20px , it will stay 20px no matter what (unless overridden by other styles or media queries).

This makes px predictable and easy to use, especially when you need exact control over layout dimensions.

However, this also means:

• Users can't easily zoom text sized in px in some browsers (though most modern ones scale the whole page now).
• Accessibility might be affected if users rely on browser-level zoom adjustments.

How rem works

rem stands for "root em." Unlike em , which is relative to its nearest parent with a defined font size, rem always refers back to the root element ( html tag) font size.

If your HTML starts like this:

 html {
  font-size: 16px;
}

Then setting:

 .title {
  font-size: 2rem;
}

Means .title will be 32px (because 2 × 16 = 32). But here's the key: you can change everything just by adjusting that one root value .

Benefits include:

• Easier site-wide scaling (great for responsive design)
• Better accessibility since users who adjust browser defaults will see proportional changes

When to use which?

Use px when:

• You want strict control over exact sizes (eg, borders, small icons)
• You don't plan to support dynamic scaling or custom user preferences at all

Use rem when:

• You're building a responsive site
• You care about accessibility and consistent scaling
• You want to make global style changes easier (like switching from desktop to mobile view)

One common practice is using rem for font sizes and layout spacing, and px only where precision matters most, like stroke widths or tiny UI details.

A few quick notes

• Some developers set html { font-size: 62.5%; } to make 1rem = 10px , simplifying math. Just remember this is a convention, not a rule.
• Mixing rem and px is totally fine — many sites do it without issues.
• Always test how your layout behaves when users change their browser's default font size.

基本上就這些。

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