diff --git a/css-forms-1/Overview.bs b/css-forms-1/Overview.bs
index 69676625a695..57e748cd406f 100644
--- a/css-forms-1/Overview.bs
+++ b/css-forms-1/Overview.bs
@@ -894,11 +894,15 @@ input[type=radio]::checkmark {
     content: '';
 }
 
-input[type=checkbox]:not([switch], :checked)::checkmark {
+input[type=checkbox]:not([switch], :checked, :indeterminate)::checkmark {
 input[type=radio]:not(:checked)::checkmark {
     visibility: hidden;
 }
 
+input[type=checkbox]:indeterminate::checkmark {
+    content: '\2500' / '';
+}
+
 input[type=radio] {
     border-radius: 50%;
 }