[css3-text] Capital spacing

Given the computed properties

     { text-transform: uppercase; letter-spacing: normal },

may a user agent apply all-caps spacing between grapheme clusters? Does 
the permission to do so depend on the fonts' provision of such metrics? 
May a user agent, in the absence of all-caps tracking metrics, adjust 
the inter-grapheme-cluster spacing based on heuristics?

-- 
Etan Wexler.