diff --git a/haddock-api/src/Haddock/Backends/Xhtml/DocMarkup.hs b/haddock-api/src/Haddock/Backends/Xhtml/DocMarkup.hs
index e63667b08c..28e3d5022c 100644
--- a/haddock-api/src/Haddock/Backends/Xhtml/DocMarkup.hs
+++ b/haddock-api/src/Haddock/Backends/Xhtml/DocMarkup.hs
@@ -167,7 +167,7 @@ hackMarkup fmt' h' =
         let id_ = makeAnchorId $ "ch:" ++ fromMaybe "noid:" nm ++ show n
             col' = collapseControl id_ "caption"
             summary = thesummary ! [ theclass "hide-when-js-enabled" ] << "Expand"
-            instTable contents = collapseDetails id_ DetailsClosed (summary +++ contents)
+            instTable contents = collapseDetails id_ DetailsOpen (summary +++ contents)
             lvs = zip [1 .. ] [h1, h2, h3, h4, h5, h6]
             getHeader = fromMaybe caption (lookup lvl lvs)
             subCaption = getHeader ! col' << markup fmt titl
diff --git a/html-test/ref/Bug335.html b/html-test/ref/Bug335.html
index f21850455e..633ed4a8c7 100644
--- a/html-test/ref/Bug335.html
+++ b/html-test/ref/Bug335.html
@@ -67,7 +67,7 @@
 	  ><div class="doc"
 	  ><h3 class="caption details-toggle-control details-toggle" data-details-id="ch:f0"
 	    >ExF:</h3
-	    ><details id="ch:f0"
+	    ><details id="ch:f0" open="open"
 	    ><summary class="hide-when-js-enabled"
 	      >Expand</summary
 	      ><p
@@ -85,7 +85,7 @@
 	  ><div class="doc"
 	  ><h3 class="caption details-toggle-control details-toggle" data-details-id="ch:g0"
 	    >ExG:</h3
-	    ><details id="ch:g0"
+	    ><details id="ch:g0" open="open"
 	    ><summary class="hide-when-js-enabled"
 	      >Expand</summary
 	      ><pre class="screen"