用于文档说明，有自带收缩、展开功能
<!DOCTYPE HTML> <html> <body> <details> <summary>HTML 5</summary> This document teaches you everything you have to learn about HTML 5. </details> </body> </html>
http://www.w3school.com.cn/tiy/t.asp?f=html5_summary
原文：http://www.cnblogs.com/tujia/p/6015398.html