remove empty attribute values

lib/single-file/serializer.js

@@ -106,13 +106,15 @@ this.serializer = this.serializer || (() => {
 			} else {
 				content += attribute.name;
 			}
-			content += "=";
-			if (!validUnquotedValue) {
-				content += "\"";
-			}
-			content += value;
-			if (!validUnquotedValue) {
-				content += "\"";
+			if (value != "") {
+				content += "=";
+				if (!validUnquotedValue) {
+					content += "\"";
+				}
+				content += value;
+				if (!validUnquotedValue) {
+					content += "\"";
+				}
 			}
 		});
 		content += ">";