1. Load data:text/html,aaa <details open>bbb <summary>ccc</summary> ddd</details> eee
2. Select all text (⌘-A / Ctrl-A)
3. Copy (⌘-C / Ctrl-C)
4. Paste on a plain text box
The pasted text should be "aaa ccc bbb ddd eee" because it's the visual order.
The pated test is "aaa bbb ccc ddd eee".
I don't think this is so severe.
Reforming tree: Pushed this down to the flattened tree family.