Definition and Usage
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).
Model: How is the z-index determined?
z-index
<div id=A> Auto 1
<div id=B> Auto 1.1
<div id=C style="z-index:1"></div> Manual 1
<div id=D></div> Auto 1.1.2
</div>
<div id=E></div> Auto 1.2
</div>
<div id=F></div> Auto 2