[css3-images] Resolution units

I know resolution defined as dots per length has already been introduced with Media Queries, but I still think it’s wrong to use ‘dpi’, ‘dpcm’ and (new) ‘dppx’ when we already have a whole bunch of length units which are just the inverse. I suggest we describe the dot size instead, using the units CSS has, or we use ‘calc()’.

  dot-count: 72dpi;
  dot-count: 100dpcm; /* 254dpi */
  dot-count: 4dppx;

became

  dot-size: 1pt;
  dot-size: 0.1mm; /* 100um */
  dot-size: 0.25px;

or

  dot-count: calc(72 / 1in); /* or (1 / 1pt) */
  dot-count: calc(10 / 1mm);
  dot-count: calc(4 / 1px); 

(I think plain numbers are fine here, but perhaps you would need to introduce a unit ‘dot’ to work well with ‘calc()’.)