Problem is probably the addressing to your css file...
For instance - the address from the main page to your css file is
@import url(_code/L2.css);
That tells the browser to look in whatever folder the current script is in for a folder called _code and then a file called L2.css
So the css document would be in
ati/site/_code/L2.css
which exists.
However when you go to another page such as
http://www.pro-creative.com/ati/site/classes/index.html
The addressing to the css file is the same.
@import url(_code/L1.css);
So you are telling the browser to look in
ati/site/classes/_code/L1.css
for the css file which does not exist.
You could either put the css file there or put a ../ in front of the addressing which would tell the browser to go up one folder in the hierarchy to then look in
ati/site/_code/L1.css
for the file.
Why this works in Safari is beyond me as it shouldn't work in any browser with this css addressing.
Abraham