diff --git a/src/css/custom.css b/src/css/custom.css
index 2527fb0dfd77fbe60c8750dbb1455e89f1409477..faff9009d1336da96eaa92e65e0a82d15ebcd568 100644
--- a/src/css/custom.css
+++ b/src/css/custom.css
@@ -14,3 +14,7 @@ dl dt {
 .center {
   text-align: center;
 }
+
+img {
+  color: red; /* color of alt text for images with invalid src url */
+}