CSS: “box-shadow”: X is not a “color” value.

The box-shadow property applies one or more shadow effects to an element. Its syntax accepts several values in a flexible order:

box-shadow: <offset-x> <offset-y> <blur-radius>? <spread-radius>? <color>? inset?;

The <color> component is optional — if omitted, it defaults to currentcolor. However, when a color is provided, it must be a valid CSS color value. The validator raises this error when it encounters something in the color position that doesn’t match any recognized color format.

Common causes of this error include:

• Misspelled color names — e.g., greyy instead of grey, or balck instead of black.
• Invalid hex codes — e.g., #GGG or #12345 (hex codes must be 3, 4, 6, or 8 hex digits).
• Fabricated color names — e.g., banana or darkwhite, which are not part of the CSS named colors specification.
• Malformed function syntax — e.g., rgb(0,0,0,0.3) using the legacy comma syntax with four values instead of rgba(), or missing parentheses.
• Incorrect value order — placing values in an unexpected position can cause the validator to misinterpret a non-color value as a color attempt.

This matters for standards compliance because browsers may silently ignore an invalid box-shadow declaration entirely, meaning your intended shadow effect won’t render. Using valid CSS ensures consistent behavior across browsers and passes validation checks.

Recognized CSS color formats

The following formats are valid for the color component:

• Named colors: red, blue, transparent, currentcolor, etc.
• Hex codes: #000, #0000, #000000, #00000080
• RGB/RGBA: rgb(0, 0, 0), rgba(0, 0, 0, 0.5), or the modern rgb(0 0 0 / 50%)
• HSL/HSLA: hsl(0, 0%, 0%), hsla(0, 0%, 0%, 0.5), or hsl(0 0% 0% / 50%)

Examples

Incorrect — misspelled or invalid color values

<!-- "balck" is not a valid color name -->

<div style="box-shadow: 2px 4px 8px balck;">Typo in color</div>

<!-- "banana" is not a recognized CSS color -->

<div style="box-shadow: 2px 4px 8px banana;">Invalid color name</div>

<!-- "#12345" is not a valid hex code (5 digits) -->

<div style="box-shadow: 2px 4px 8px #12345;">Bad hex code</div>

Correct — valid color values

<!-- Named color -->

<div style="box-shadow: 2px 4px 8px black;">Named color</div>

<!-- Hex color -->

<div style="box-shadow: 2px 4px 8px #333;">Hex color</div>

<!-- RGBA for semi-transparency -->

<div style="box-shadow: 2px 4px 8px rgba(0, 0, 0, 0.3);">Semi-transparent</div>

<!-- HSL color -->

<div style="box-shadow: 2px 4px 8px hsl(210, 50%, 40%);">HSL color</div>

Correct — omitting the color entirely

If you want the shadow to inherit the element’s text color, you can omit the color value altogether. This is valid and avoids the error:

<!-- Defaults to currentcolor -->

<div style="box-shadow: 2px 4px 8px;">Uses currentcolor</div>

Correct — multiple shadows with valid colors

<div style="box-shadow: 2px 2px 4px rgba(0, 0, 0, 0.2), inset 0 0 6px #ccc;">
  Multiple shadows
</div>

To resolve this validation error, check the exact value the validator flags (the “X” in the error message), verify its spelling against the list of CSS named colors, and ensure any hex or function-based color uses correct syntax. If the color isn’t needed, simply remove it and let the browser default to currentcolor.

Learn more:

MDN Reference

MDN Web Docs: box-shadow

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MDN Reference

CSS color value

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MDN Reference

CSS box-shadow

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