Hi Tim,
In terms of the css, I should have been more clear. When I change the margin values in the css you specify to zero, the left margin remains indented, as if its inheriting some indent from somewhere else. Is it perhaps just my browser?
Seems to be your browser.
On Linux/KDE Firefox and Konqueror it renders fine :
The code-box is left-justified with the normal content (no indenting), an we have an indent of 4ex on the right.
Eberhard