diff --git a/README.md b/README.md
index ae0e884..1cdbd54 100644
--- a/README.md
+++ b/README.md
@@ -13,3 +13,10 @@ Blog posts live in the `posts/` directory. Open a post by visiting
 
 No build step is required. Simply open the files directly in your browser.
 
+If your browser shows "Error loading post." when loading a blog post directly from the file system, start a local server:
+
+    python3 -m http.server
+
+Then visit http://localhost:8000/blog.html?post=hello-world (or any slug). This avoids cross-origin restrictions that can block `fetch` from `file:` URLs.
+
+