CSS variables improve the neatness and maintenance of the code by storing reusable values ??and achieving one modification and multiple modifications. Its core role is to simplify stylesheet management and support dynamic updates and local scope control. When using it, you need to define variables with the -- prefix, call them through the var() function, pay attention to naming specifications and scope division, and be wary of spelling errors and compatibility issues. The specific points are as follows: 1. Use --prefix to define variables, usually declare global variables in: root; 2. Reference variables through the var() function; 3. Any CSS value such as color, size, shadow, etc. can be stored; 4. Local variables should be defined in a specific selector to avoid conflicts; 5. Dynamic updates can be achieved through JavaScript or media queries; 6. Pay attention to spell checking, inheritance behavior and browser compatibility issues. The rational use of CSS variables can significantly improve development efficiency and code maintainability.

Using CSS variables (also known as custom properties) can make your stylesheets cleaner, more maintainable, and easier to update. They let you store reusable values—like colors, font sizes, spacing—and change them in one place instead of hunting through your code.

What Are CSS Variables?

CSS variables are defined using the -- prefix inside a selector, usually :root for global scope. You access them with the var() function.

 :root {
  --primary-color: #007bff;
  --spacing: 1rem;
}

Then use them like this:

 .button {
  background-color: var(--primary-color);
  padding: var(--spacing);
}

They're not just for colors—you can store anything: sizes, shadows, font stacks, even complex gradients.

How to Scope Variables Locally

By default, defining variables on :root makes them globally available. But sometimes you want variables that only apply in certain parts of the site—like a dark theme or a specific component.

Just define them on a selector:

 .card {
  --bg-color: #f8f8f8;
  background-color: var(--bg-color);
}

Now, only elements under .card will see that value. This helps avoid naming conflicts and keeps things modular.

Tips:

• Use meaningful names ( --header-padding , not --p1 )
• Keep local variables tightly scoped
• Don't overdo it—some values ??are fine being hard-coded

Updating Variables Dynamically

One of the big wins with CSS variables is being able to change them at runtime—say, switching themes or adjusting layout based on user input.

You can do this in JavaScript:

 document.documentElement.style.setProperty('--primary-color', '#ff0000');

Or with media queries:

 @media (max-width: 600px) {
  :root {
    --spacing: 0.5rem;
  }
}

This flexibility makes responsive design and theming easier without needing extra classes or preprocessors.

Common Pitfalls and Gotchas

Even though CSS variables are powerful, there are a few things to watch out for:

• No fallback if misspelled : If you typo a variable name, like var(--primry-color) , it won't work and won't warn you.
• Cascading behavior matters : Variables inherit like normal properties, so changing a parent's variable affects children.
• Old browser support : Most modern browsers support them, but not IE11.

To stay safe:

• Double-check variable names
• Use fallbacks when needed: color: var(--text-color, black);
• Consider tools like PostCSS if you need broader compatibility

That's the core of working with CSS variables. They're not complicated, but they do require attention to naming and scope. Once you get used to them, you'll probably wonder how you ever managed without.

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