diff --git a/sinatra_app.rb b/sinatra_app.rb
index fba6dee..b252603 100644
--- a/sinatra_app.rb
+++ b/sinatra_app.rb
@@ -9,5 +9,17 @@
 set :port, options[:port] || 3001
 
 get "/" do
-  "hello"
+  <<~HTML
+    <!doctype html>
+    <html lang="en">
+      <head>
+        <meta charset="utf-8">
+        <meta name="viewport" content="width=device-width, initial-scale=1">
+        <title>Preview</title>
+      </head>
+      <body style="margin:0;min-height:100vh;display:flex;align-items:center;justify-content:center;background:#ff0000;color:#ffffff;font-family:system-ui,sans-serif;">
+        hello
+      </body>
+    </html>
+  HTML
 end
