使用rem單位更適合全局布局和一致性設(shè)計，1.rem基于根元素字體大小，確保統(tǒng)一性；2.避免嵌套時的尺寸疊加問題；3.適合設(shè)置全局間距和布局；4.em適用于需要局部縮放的場景，如按鈕、標(biāo)簽或注重排版比例的設(shè)計。

Using rem and em units in CSS both have their place, but choosing between them depends on what you're trying to achieve with your layout and typography. Here’s a breakdown of when it makes more sense to go with rem.

When consistency across the page matters

rem stands for "root em," which means it's always based on the font size of the HTML element (the root). This makes it predictable. If you set your base font size on the html element — say, html { font-size: 16px; } — then 1rem is always equal to that base value, no matter where you use it.

This is especially useful if you want consistent spacing, padding, or font sizes throughout your site. With em, those values can change depending on the context — like nested elements — which can get confusing fast.

So if you're building a design system or need uniformity in spacing and sizing, rem is the safer bet.

When avoiding compounding issues is important

One big gotcha with em is that it scales relative to the font size of the current element — and that includes any parent elements. That means if you nest multiple elements using em, the sizes can compound unintentionally.

For example:

• Parent has font-size: 2em;
• Child has padding: 1em;
• The child's padding ends up being 2em relative to the parent, not the root

With rem, this issue disappears because everything is tied back to the root font size. No surprises from nesting.

When setting global styles or layout spacing

If you're defining things like margins, padding, or grid gaps at a high level — maybe inside a CSS reset or a utility class system — rem helps keep spacing aligned with your overall design scale.

You’ll often see systems using something like 1rem = 10px or 1.25rem = 20px as a base unit for spacing. That makes math easier and keeps your layout modular without having to constantly calculate inherited sizes.

When to still use em

Of course, there are times when em shines:

• When you do want scaling behavior based on local font size — like buttons or labels that should grow with the text
• In print stylesheets or accessibility-focused designs where users might override font sizes
• For vertical rhythm techniques where proportional scaling matters

But if you're aiming for control and predictability across your site, rem gives you a solid foundation.

In most modern web projects, especially ones that prioritize maintainable and scalable styles, rem tends to be the preferred choice for layout and spacing. em still has its uses, but they’re more situational.

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