diff --git a/treescope/lowering.py b/treescope/lowering.py
index cdfa44c..bc17793 100644
--- a/treescope/lowering.py
+++ b/treescope/lowering.py
@@ -321,6 +321,7 @@ def _render_one(
     classnames += " roundtrip_mode"
   stream.write(
       f'<div class="{classnames}" tabindex="0" '
+      'part="treescope_root" '
       'onkeydown="this.getRootNode().host.defns'
       '.toggle_root_roundtrip(this, event)">'
   )