[CSS3-MQ] Resolution

CSS3 Media Queries CR introduces two new units (7.), |dpi| and | 
dpcm|, solely for its |resolution| media feature (6.9.). Could this  
be changed or at least be amended to also allow standard CSS absolute  
length units (e.g. |mm|)? The values using |in| and |cm| would of  
course be the respective reciprocals of the current ones and  
described the size of one point^W pixel. If a new unit was needed at  
all this would be |um|, the ASCII representation of 'µm' for  
micrometre, approved by ISO (31?).
It might also be a good idea to make this media feature a shorthand  
for horizontal and vertical resolution for which separate features  
might be added later.