<!doctype html> <meta charset="utf-8"> <title>CSS Test Reference</title> <style> summary { display: list-item; counter-increment: list-item 0; list-style: disclosure-closed inside; } details[open] > summary { list-style-type: disclosure-open; } </style> <details> <summary>Example</summary> </details> <details open> <summary>Example</summary> </details>