The color space used for calculations.
Browsers mostly implement this consistantly (after some recent work).
No browsers support color-interpolate="linearRGB"
Up/down scaling should be done in linearRGB:
A good discussion at Gamma error in picture scaling web site.
Gimp now uses 16 bit color, scaling now done correctly. Photoshop also does the correct thing with 16/32 bit color. Interest is increasing in getting better color support: Xbox, Google Image Correction.
Historicaly, sRGB space used for performance reasons. With more power computers and off-loading to GPU's, we should at least be able to specify that the browsers do the right thing.