diff --git a/docs/demo/demo.css b/docs/demo/demo.css
index 66b870adad..92d2f4a504 100644
--- a/docs/demo/demo.css
+++ b/docs/demo/demo.css
@@ -45,10 +45,6 @@ header h1 {
 	box-sizing: border-box;
 }
 
-#options {
-	resize: both;
-}
-
 .pane, .inputPane {
 	margin-top: 5px;
 	padding: 0.6em;
@@ -65,3 +61,8 @@ header h1 {
 #preview iframe {
 	flex-grow: 1;
 }
+
+#options.badParse {
+	border-color: red;
+	background-color: #FEE
+}
diff --git a/docs/demo/demo.js b/docs/demo/demo.js
index 09b30dc204..7da5fe3b9a 100644
--- a/docs/demo/demo.js
+++ b/docs/demo/demo.js
@@ -181,7 +181,9 @@ function checkForChanges() {
       var optionsString = $optionsElem.value || '{}';
       var newOptions = JSON.parse(optionsString);
       options = newOptions;
+      $optionsElem.classList.remove('badParse');
     } catch (err) {
+      $optionsElem.classList.add('badParse');
     }
 
     var lexed = marked.lexer($markdownElem.value, options);