--- a/devtools/client/themes/rules.css
+++ b/devtools/client/themes/rules.css
@@ -315,16 +315,25 @@
 }
 
 .ruleview-enableproperty {
   position: relative;
   float: left;
   left: -28px;
 }
 
+.ruleview-warning {
+  background-image: url(images/alerticon-warning.png);
+  background-size: 13px 12px;
+  margin-inline-start: 5px;
+  display: inline-block;
+  width: 13px;
+  height: 12px;
+}
+
 .ruleview-rule:not(:hover) .ruleview-enableproperty {
   visibility: hidden;
 }
 
 .ruleview-expander {
   vertical-align: middle;
 }