avoid processing unused fonts if none found

lib/single-file/css-fonts-minifier.js

@@ -74,13 +74,15 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 				const filteredUsedFonts = new Set(usedFonts.map(fontNames => fontNames.find(fontName => declaredFonts.has(fontName))).filter(fontName => fontName));
 				unusedFonts = Array.from(declaredFonts).filter(fontFamilyName => !filteredUsedFonts.has(fontFamilyName));
 			}
-			doc.querySelectorAll("style").forEach(style => {
-				if (style.sheet) {
-					const processedRules = style.sheet.cssRules.length;
-					style.textContent = deleteUnusedFonts(doc, style.sheet.cssRules, unusedFonts);
-					stats.rules.discarded += processedRules - style.sheet.cssRules.length;
-				}
-			});
+			if (unusedFonts.length) {
+				doc.querySelectorAll("style").forEach(style => {
+					if (style.sheet) {
+						const processedRules = style.sheet.cssRules.length;
+						style.textContent = deleteUnusedFonts(doc, style.sheet.cssRules, unusedFonts);
+						stats.rules.discarded += processedRules - style.sheet.cssRules.length;
+					}
+				});
+			}
 			return stats;
 		}
 	};