CSS變量的回退值通過var()函數(shù)的第二個(gè)參數(shù)設(shè)置，用于在變量未定義或無效時(shí)提供默認(rèn)值。例如，在.box { background-color: var(--main-color, #ccc); }中，若--main-color未定義或無效，則背景色會(huì)使用#ccc?；赝酥颠m用于各種CSS值類型，如長(zhǎng)度、關(guān)鍵字及嵌套變量，例如：1. 默認(rèn)間距 padding: var(--spacing, 1rem);；2. 字體棧 font-family: var(--heading-font, sans-serif);；3. 嵌套回退 color: var(--text-color, var(--default-text-color, black));。但需注意，當(dāng)變量已定義但類型不合法時(shí)（如--my-width: red; width: var(--my-width, 50%);），回退機(jī)制不會(huì)生效。最佳實(shí)踐包括：在:root中定義基礎(chǔ)變量、使用語義化命名、保持回退值簡(jiǎn)潔、測(cè)試變量缺失場(chǎng)景，并可將回退值設(shè)為其他變量以構(gòu)建主題層級(jí)。

You can set a fallback value for a CSS variable by using the second parameter in the var() function. This is super useful when you want to ensure a default value kicks in if the variable isn't defined or is invalid.

Why You Might Want to Use a Fallback

Sometimes, your CSS variables might not be defined due to scope issues, typos, or loading order problems. Rather than having styles break unexpectedly, a fallback gives you control over what happens in those cases.

For example:

:root {
  /* --main-color might be missing or invalid */
}

.box {
  background-color: var(--main-color, #ccc);
}

In this case, if --main-color doesn’t exist or is invalid, .box will use #ccc as its background color.

How to Apply Fallbacks in Different Contexts

Fallback values aren’t just for colors — they work with any kind of CSS value, like lengths, keywords, or even other variables.

Here are some common use cases:

• Default spacing

  padding: var(--spacing, 1rem);

• Fallback font stack

  font-family: var(--heading-font, sans-serif);

• Nested fallback (using another variable)

  color: var(--text-color, var(--default-text-color, black));

This kind of nesting lets you build more robust themes and component systems.

When Fallbacks Won’t Work

It’s worth noting that fallbacks only help inside var(). If you're not using the var() syntax at all, there's no way for the browser to fall back. Also, the fallback won’t apply if the variable exists but has an invalid property-specific value.

For instance:

width: var(--my-width, 50%); /* works if --my-width is undefined or invalid */

But if you do something like this:

--my-width: red;
width: var(--my-width, 50%); /* invalid for 'width', but still resolves to 'red' */

The browser will try to use red as a width, which doesn’t make sense — but it won't fall back to 50%.

So, fallbacks don't protect against incorrect types — just missing or malformed variables.

Best Practices for Using Fallbacks

• Always define base variables on :root to avoid scoping issues.
• Use semantic names so it's clear what the fallback is for.
• Keep fallbacks simple unless you're intentionally building layered defaults.
• Test scenarios where a variable might be missing — especially in larger projects or design systems.

And one more tip: fallbacks can also be custom properties themselves, which is handy when organizing theme layers or providing dynamic defaults.

Basically, it’s a small feature that adds a lot of flexibility without much overhead.

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