packages/happy-dom/src/css/declaration/element-style/CSSStyleDeclarationElementStyle.ts

@@ -19,7 +19,7 @@ import CSSMeasurementConverter from '../measurement-converter/CSSMeasurementConv
 import MediaQueryList from '../../../match-media/MediaQueryList.js';
 import WindowBrowserSettingsReader from '../../../window/WindowBrowserSettingsReader.js';
 
-const CSS_VARIABLE_REGEXP = /var\( *(--[^) ]+)\)/g;
+const CSS_VARIABLE_REGEXP = /var\( *(--[^), ]+)\)|var\( *(--[^), ]+), *([^), ]+)\)/g;
 const CSS_MEASUREMENT_REGEXP = /[0-9.]+(px|rem|em|vw|vh|%|vmin|vmax|cm|mm|in|pt|pc|Q)/g;
 
 type IStyleAndElement = {
@@ -333,7 +333,12 @@ export default class CSSStyleDeclarationElementStyle {
 		let match;
 
 		while ((match = regexp.exec(value)) !== null) {
-			newValue = newValue.replace(match[0], cssVariables[match[1]] || '');
+			// Fallback value - E.g. var(--my-var, #FFFFFF)
+			if (match[2] !== undefined) {
+				newValue = newValue.replace(match[0], cssVariables[match[2]] || match[3]);
+			} else {
+				newValue = newValue.replace(match[0], cssVariables[match[1]] || '');
+			}
 		}
 
 		return newValue;

packages/happy-dom/test/window/BrowserWindow.test.ts

@@ -487,6 +487,45 @@ describe('BrowserWindow', () => {
 			expect(computedStyle.color).toBe('');
 		});
 
+		it('Returns values defined by a CSS variables when a fallback is used.', () => {
+			const parent = <IHTMLElement>document.createElement('div');
+			const element = <IHTMLElement>document.createElement('span');
+			const computedStyle = window.getComputedStyle(element);
+			const parentStyle = document.createElement('style');
+			const elementStyle = document.createElement('style');
+
+			browserFrame.page.setViewport({ width: 1024 });
+
+			parentStyle.innerHTML = `
+                html {
+                    font: 14px "Times New Roman";
+                }
+
+				div {
+					--border-variable: 1px solid var(--color-variable, #000);
+					--font-variable: 1rem "Tahoma";
+				}
+			`;
+
+			elementStyle.innerHTML = `
+				span {
+					border: var(--border-variable);
+					font: var(--font-variable);
+                    color: var(--invalid-variable);
+				}
+			`;
+
+			parent.appendChild(elementStyle);
+			parent.appendChild(element);
+
+			document.body.appendChild(parentStyle);
+			document.body.appendChild(parent);
+
+			expect(computedStyle.border).toBe('1px solid #000');
+			expect(computedStyle.font).toBe('14px "Tahoma"');
+			expect(computedStyle.color).toBe('');
+		});
+
 		it('Returns a CSSStyleDeclaration object with computed styles containing "rem" and "em" measurement values converted to pixels.', () => {
 			const parent = <IHTMLElement>document.createElement('div');
 			const element = <IHTMLElement>document.createElement('span');