diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index ab7eb59774..fb4eee8f0e 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -9359,6 +9359,12 @@ private:
       case 0x00A0:
         aOut.AppendLiteral(" ");
         break;
+      case '<':
+        aOut.AppendLiteral("&lt;");
+        break;
+      case '>':
+        aOut.AppendLiteral("&gt;");
+        break;
       default:
         aOut.Append(*c);
         break;
@@ -9500,6 +9506,12 @@ AppendEncodedAttributeValue(nsAutoString* aValue, StringBuilder& aBuilder)
       case 0x00A0:
         extraSpaceNeeded += ArrayLength("&nbsp;") - 2;
         break;
+      case '<':
+        extraSpaceNeeded += ArrayLength("&lt;") - 2;
+        break;
+      case '>':
+        extraSpaceNeeded += ArrayLength("&gt;") - 2;
+        break;
       default:
         break;
     }