packages/happy-dom/src/css/declaration/property-manager/CSSStyleDeclarationValueParser.ts

@@ -262,7 +262,7 @@ export default class CSSStyleDeclarationValueParser {
 	 * @returns Parsed value.
 	 */
 	public static getMeasurement(value: string): string {
-		return this.getLength(value) || this.getPercentage(value);
+		return this.getLength(value) || this.getPercentage(value) || this.getCalc(value);
 	}
 
 	/**

packages/happy-dom/test/css/declaration/CSSStyleDeclaration.test.ts

@@ -2103,6 +2103,14 @@ describe('CSSStyleDeclaration', () => {
 
 			expect(declaration.width).toBe('0px');
 		});
+
+		it('Returns width for "width: calc(10% - 1px)".', () => {
+			const declaration = new CSSStyleDeclaration(element);
+
+			element.setAttribute('style', 'width: calc(10% - 1px)');
+
+			expect(declaration.width).toBe('calc(10% - 1px)');
+		});
 	});
 
 	describe('get height()', () => {