Elements can be placed in front of or behind other elements according to the Z index value.
The value should be defined with an integer or can be replaced by the auto keyword. The auto value is the same as setting this property to zero.
Any elements that share the same zIndex value will be stacked in the display using their order as presented in the document source.
The CSS-P standard name for the property is z-index so this property may be accessible with a different name on some implementations.