/**
 * Validates that a string is a safe CSS length value before injecting it
 * into a CSS custom property. Accepts standard length units, percentages,
 * CSS keywords, and `calc()`/`var()` expressions without embedded semicolons,
 * curly braces, or other CSS injection vectors.
 *
 * @returns The trimmed value if safe, `null` otherwise.
 */
export declare function safeCSSLength(value: string | null | undefined): string | null;
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