CSS: “margin-bottom”: “spacing(N)” is not a “margin-bottom” value.

The spacing() function is not a valid CSS value. It does not exist in any CSS specification.

The spacing() notation is commonly found in utility-based frameworks like Tailwind CSS or custom design systems where it acts as a shorthand for spacing scales. However, browsers and the W3C validator do not recognize it as valid CSS.

If you’re using a build tool or preprocessor, make sure spacing() is being compiled into a valid CSS value before it reaches the browser. If you’re writing plain CSS, replace it with a standard CSS length value such as px, rem, em, or use a CSS custom property.

How to Fix

Replace spacing(3) with a valid CSS length value:

#content::after {
  margin-bottom: 0.75rem;
}

Or use a CSS custom property to create your own spacing scale:

:root {
  --spacing-3: 0.75rem;
}

#content::after {
  margin-bottom: var(--spacing-3);
}

Both approaches produce valid CSS that the W3C validator will accept. If your project relies on a framework that provides spacing(), check that your build pipeline (e.g., PostCSS, Sass, or Tailwind) is correctly transforming it into standard CSS before deployment.

Learn more:

MDN Reference

margin-bottom - CSS: Cascading Style Sheets | MDN

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MDN Reference

CSS values and units | MDN

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