diff --git a/doc/book.toml b/doc/book.toml
index 741aacf707..8ff0a54334 100644
--- a/doc/book.toml
+++ b/doc/book.toml
@@ -13,5 +13,9 @@ git-repository-url = "https://github.com/leanprover/lean4"
 additional-css = ["alectryon.css", "pygments.css"]
 additional-js = ["alectryon.js"]
 
+[output.html.fold]
+enable = true
+level = 0
+
 [output.html.playground.boring-prefixes]
 lean = "# "