Q: CSS Background parameters trouble background : url(../images/li.png) no-repeat 0 50%; 

what parameters are 0 and 50%? they are not specified explicitly 

A: This is the position.
0 50% would be left middle (0% on left-position, 50% of top-position).
You can check the spec for more info : http://www.w3.org/TR/CSS2/colors.html

A: The background property is shorthand for:


*

*background-color

*background-image

*background-repeat

*background-attachment

*background-position
The 0 50% is the background-position.
See this awesome website for details: http://www.htmldog.com/reference/cssproperties/background/
