[css3-background] background-size and zero length

http://dev.w3.org/csswg/css3-background/#the-background-size
# Negative values are not allowed.

div
{
        background-image: url(http://www.microsoft.com/favicon.ico) 5px 5px;
        background-size: 0px 20px;
        background-repeat:round;
        width:12px;
        height:17px;
}

How many images should be rendered and at what size?

* Brian