If you only had to worry about Firefox and Webkit browsers, what CSS would you use to make the footer in the following HTML not rise above the bottom or the screen (and go lower
I think you're talking about the margin of 10-15 pixels below the footer. Try adding this code to your CSS;
body { margin-bottom:0px; }
The body has a default margin of 10-15 pixels so in order to remove it to flush your design to the top or bottom you have to tell the body to have a margin of zero.
Hope that helps.