removed unneeded variable

lib/single-file/serializer.js

@@ -49,15 +49,13 @@ this.serializer = this.serializer || (() => {
 	}
 
 	function serialize(node) {
-		let content = "";
 		if (node.nodeType == Node.TEXT_NODE) {
-			content += serializeTextNode(node);
+			return serializeTextNode(node);
 		} else if (node.nodeType == Node.COMMENT_NODE) {
-			content += serializeCommentNode(node);
+			return serializeCommentNode(node);
 		} else if (node.nodeType == Node.ELEMENT_NODE) {
-			content += serializeElement(node);
+			return serializeElement(node);
 		}
-		return content;
 	}
 
 	function serializeTextNode(textNode) {