z-index property profile
| z-index: auto | number
z-index value will be superimposed on the larger elements of z-index value on the smaller elements. The positioning of this property is not specified, the object, z-index value is the number of objects in its top, while the negative z-index value of an object under his.
Note: This property does not act on the window controls, such as selct the object. In IE 5.5 + in, iframe object began to support this attribute. In the previous browser versions, iframe object is a window control, will ignore this case
z-index attribute applies to positioned elements (position attribute value of relative or absolute or fixed object), is used to determine the positioning elements in the direction perpendicular to the screen (called the Z axis) on the stacked sequence (stack order).
Position property of each element belongs to a stacking context. Root element of the form root stacking context, while the other stacking context created by the positioning elements (elements of this positioning z-index is defined as a non-auto's z-index value), position child elements will be the local stacking context as a reference, with the same rules to determine the cascading order. And stacking context and the containing block is not associated with between.
When stacking context the same time, the use of z-index value to determine how to display, if the z-index are the same (that is, the same stack level), then according to those who file later on the principle of home (back-to-front) of the in order to stack.
When any one element stack to another included in the different stacking context elements will be stacked with stacking context of the level (stack level) to determine the sequence of display cases. In other words, the same stacking context under which it will use the z-index to determine successively, not simultaneously by stacking context of the z-index to decide. For example:
Positioning element A (z-index: 100) which has positioning elements A1 (z-index: 300), while the positioning element B and element A brotherhood (z-index: 200). A1 you will find both the z-index is how much will be z-index is a B-200 is covered, because A, z-index is only 100.
Read more detailed content: http://www.w3.org/TR/CSS21/visuren.html # z-index