The details Element HTML5

This new element helps mark up a section of the document that’s hidden, but can
be expanded to reveal additional information. The aim of the element is to provide
native support for a feature common on the Web—a collapsible box that has a title,
and more info or functionality hidden away.
Normally this kind of widget is created using a combination of markup and scripting.
The inclusion of it in HTML5 intends to remove the scripting requirements and
simplify its implementation for web authors.

<summary>Some Magazines of Note</summary>
<li><cite>Bird Watchers Digest</cite></li>
<li><cite>Rowers Weekly</cite></li>
<li><cite>Fishing Monthly</cite></li>

If details lacks a defined summary, the user agent will define a default summary
(for example, “Details”). If you want the hidden content to be visible by default,
you can use the Boolean open attribute.