> Which HTTRack version are you using ?
Version 3.33 (the last one, I think)

> Did you modify options in set Options / Build / Local
> Structure Type ?
No, the default option is set ("site-structure"). Everything
is set as it was when i first installed the program

> In the css, HTTrack resolves url(foo.jpg) as relative to
>the folder where the css is which is the case.

Well, that's exactly what it DOESN'T do in my case! I wonder
which variable could explain the fact that your htttrack
works... and mine not!  
 
> Importing the css is a way to hide the style sheet from >
> old browsers 

Yes, I made some experiments and the result is the same, it
doesn't matter whether the css is imported or linked.
I'm going to scan the log files to try to understand what
happens during the download... argh!