Version:           3.5.7 (using KDE 3.5.8)
Installed from:    SuSE RPMs
OS:                Linux

CSS completion works fine between style tags but -  as far as I can see - is not available when editing external style sheets (the preferred implementation of CSS for most professional developers).  What I currently do is enclose all my css in the external file between style tags (so I get code completion), then when I'm ready to preview, I delete the style tags ..... not very elegant.  Since all the code that handles CSS completion is already in place, surely it would be a relatively quick fix to enable this module for an external style sheet.  
For an idea of what's out there (commercial product, I admit but extremely cheap) check out CSS handling in a tool called WeBuilder (www.blumentals.net).  This is by far the best HTML/PHP/CSS editor on Windows and Quanta already does most of what it does!  

Apart from that, I think Quanta is a storming environment.  Keep up the excellent work.  Thank you.