[css-forms-1] Add checkmark content for indeterminate checkboxes

Author: lukewarlow

css-forms-1/Overview.bs

@@ -891,11 +891,15 @@ input[type=radio]::checkmark {
     content: '\2713' / '';
 }
 
-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: '\2014' / '';
+}
+
 input[type=radio] {
     border-radius: 50%;
 }