The z-index property specifies the stack order of an element.
An element with greater stack order is always in front of an element with a lower stack order.
Note: z-index only works on positioned elements (position:absolute, position:relative, or position:fixed).
1 z-index 2 <div id=A> Auto 1 3 <div id=B> Auto 1.1 4 <div id=C style="z-index:1"></div> Manual 1 5 <div id=D></div> Auto 1.1.2 6 </div> 7 <div id=E></div> Auto 1.2 8 </div> 9 <div id=F></div> Auto 2
原文：http://www.cnblogs.com/hzj680539/p/5200001.html