The difference is less about the actual difference in layouting (though it is
important) its more about the different way of using. (UseCase)
A common goal the user has in mind when he adjusts the letter-spacing in
typesetting is to make it possible to make a word fit on a line, to avoid
hyphenation for example.
In percent based letter spacing effectively adjusts the width of the whole
selected region in, well, a percentage of the effective width.
So if you have a sentence that has a text width of 100 and you apply a 97%
letter spacing to it the text will end up being 97. Irregardless of the
amount of characters on the line.
I can understand to your usecase, but I don't think that such
requirement should be expressed using fo:letter-spacing as this would
break "compatibility" with XSL-FO. Moreover, your usecase seems more as
a constraint on layout algorithm which can result in modification of
interletter space, but not vice versa.
Maybe some completely new attribute like visual-compression="97" would
be more appropriate.
Note that I'm not saying that one method is good and one method is bad. Qt4
supports both, and from a user perspective I think KOffice should use the
percent based one. Its just easier to use and more in line with what users
want to achieve.
It's questionable. For example, I have never specified letter-spacing
using percentages before, I have been always specifying length directly.
Thanks for your feedback, I didn't realize there are more typography lovers on
this list ;)