diff --git a/src/elements.lisp b/src/elements.lisp
index c459b33..fe539ec 100644
--- a/src/elements.lisp
+++ b/src/elements.lisp
@@ -191,9 +191,21 @@
   style sub summary sup svg table tbody td template textarea tfoot th
   thead |time| title tr track u ul var video wbr)
 
+(defparameter *boolean-attrs*
+  '(allowfullscreen async autofocus autoplay checked controls default defer
+    disabled formnovalidate inert ismap itemscope loop multiple muted nomodule
+    novalidate open playsinline readonly required reversed selected))
+
 (defmethod print-object ((attrs attrs) stream)
   (if (attrs-alist attrs)
-      (format stream " ~{~a=~s~^ ~}" (util:alist-plist (attrs-alist attrs)))
+      (let ((alist (attrs-alist attrs)))
+        (dolist (pair alist)
+          (let ((key (car pair))
+                (value (cdr pair)))
+            (if (member key *boolean-attrs* :test #'string=)
+                (when value
+                  (format stream " ~a" (string-downcase key)))
+                (format stream " ~a=~s" (string-downcase key) value)))))
       (format stream "")))
 
 (defparameter *self-closing-tags*
diff --git a/t/piccolo.lisp b/t/piccolo.lisp
index b59cdb3..86df663 100644
--- a/t/piccolo.lisp
+++ b/t/piccolo.lisp
@@ -148,6 +148,14 @@
     (is (string= "<div id=\"4\"><div id=\"3\"><div>some text</div><div id=\"2\"></div>some other text</div><div id=\"5\"><a href=\"a.html\">a</a></div></div>"
                  (elem-str div4)))))
 
+(test boolean-attrs
+  (let ((script1 (script :defer t :data-domain "example.com" :src "example.com/script.js"))
+        (script2 (script :defer nil :data-domain "example.com" :src "example.com/script.js")))
+    (is (string= "<script defer data-domain=\"example.com\" src=\"example.com/script.js\"></script>"
+                 (element-string script1)))
+    (is (string= "<script data-domain=\"example.com\" src=\"example.com/script.js\"></script>"
+                 (element-string script2)))))
+
 (in-suite escape)
 
 (defparameter *a-attrs*