CSS: “--X”: Character Y is neither a decimal digit number, decimal point, nor "e" notation exponential mark.

The value of a custom property has a calc(), min(), max() or clamp() expression with a word where a number should be, such as undefined left behind by a script.

The name in the message is the custom property (any name starting with --), and the letter is the first letter of the word the validator found. Inside a math function it tries to read each operand as a number, so calc(undefined * 1px) reports "Character u", calc(10px + auto) reports "Character a", and calc(px * 2) reports "Character p". The constants pi, infinity and NaN are allowed there and don't trigger it.

The validator is stricter than browsers here. A custom property accepts almost any value, and browsers don't check the math inside it when they read the stylesheet, so they keep the declaration. The validator still checks math functions inside custom properties, although it lets a plain --button-min-width: undefined through without a message.

The value is broken all the same, and it shows where the variable is used. With min-width: var(--button-min-width), the browser substitutes calc(undefined * 1px), which isn't a valid length, so min-width behaves as if it were unset. A fallback such as var(--button-min-width, 120px) doesn't help: it only applies when the custom property isn't set at all.

The word is usually undefined, printed by a script or template that writes the style from a JavaScript variable that was never set. Fix the script so the variable always holds a number, or leave the declaration out when there is no value. If a theme, plugin or site builder generates the CSS, the fix belongs in its settings or with its developer. Inside the math, var() is accepted, so calc(var(--button-columns, 1) * 120px) is fine.

Invalid example

<button type="button" style="--button-min-width: calc(undefined * 1px);">Add to cart</button>

Valid example

<button type="button" style="--button-min-width: calc(120 * 1px);">Add to cart</button>

Learn more:

MDN Reference

MDN Web Docs: Custom properties (--*)

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MDN Reference

MDN Web Docs: calc()

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MDN Reference

MDN Web Docs: var()

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