In CSS2.1 , the computed value of letter-spacing is specified to be:
Computed value: 'normal' or absolute length
In contrast, the computed value of word-spacing is specified to be:
Computed value: for 'normal' the value '0'; otherwise the absolute length
There appears to be no reason why these are different, i.e., the computed value of letter-spacing should take the same formulation as for word-spacing.
The present situation presents some uncertainty for the implementation of CSSOM getComputedStyle().letterSpacing, which, by the above, would appear to dictate returning the string 'normal' instead of '0'.
It is also possibly a problem that either of these should return '0' instead of '0px', which is more consistent with returning a resolved (used) length value.
I recommend that the language of both of these properties specify computed value as follows:
Computed value: for 'normal' the value '0px'; otherwise the absolute length