Cascading Style Sheets - HTML and XHTML: The Definitive Guide - page 511

Z positions are not absolute throughout a document. Instead, z positions
are relative to the containing element. For example, two <div> elements
within a document might be positioned to lie on top of one another. The
first <div> might have a z position of 1, and the second might have a z
position of 2. The entire contents of the second <div> are displayed over
(or in front of ) the first <div> . If elements within the first <div> have z
positions of 3 or 4, they are still displayed within their containing <div> s
and do not "jump out" in front of the second <div> .
You control the z position of an element with the z-index property. The
value of the z-index property is a positive integer that sets the z position
of the element with respect to its containing element. With the z-index
property, you can dynamically alter the z position of an element to make
it visible, or position a text element in front of an image to label items
of interest.
8.4.8. List Properties
The CSS2 standard also lets you control the appearance of list ele-
mentsspecifically, ordered and unordered lists. Browsers format list
items just like any other block item, except that the block has some sort
of marker preceding the contents. For unordered lists, the marker is a
bullet of some sort; for numbered lists, the marker is a numeric or al-
phabetic character or symbol. The CSS2 list properties let you control
the appearance and position of the marker associated with a list item.
8.4.8.1. The list-style-image property
The list-style-image property defines the image that the browser uses
to mark a list item. The value of this property is the URL of an image file
or the keyword none . The default value is none .
The image is the preferred list marker. If it is available, the browser dis-
plays it in place of any other defined marker. If the image is unavailable,
or if the user has disabled image loading, the browser uses the mark-