diff --git a/docker/observer_web/dev.exs b/docker/observer_web/dev.exs
index e5af39d..0a1165c 100644
--- a/docker/observer_web/dev.exs
+++ b/docker/observer_web/dev.exs
@@ -12,8 +12,9 @@ defmodule WebDev.Router do
   scope "/" do
     pipe_through(:browser)
 
-    # Dedicated host observer.*.calentiq.com — mount at root (no /observer prefix)
-    observer_dashboard("/")
+    # Dedicated host: mount at "" not "/" — library builds "#{prefix}/css-HASH",
+    # and prefix="/" yields broken protocol-relative "//css-…".
+    observer_dashboard("")
   end
 end