Created attachment 103168 [details] demonstration If you use letter-spacing with left alignment, the first letter aligns with other left-aligned paragraphs that don't have letter-spacing set. But with right alignment, the last letter does not align with other right-aligned paragraphs: it is shifted left by the letter-spacing amount. In other words, letter-spacing currently adds the specified amount of space to the right of the glyph. I think that violates CSS2, which says that "This property specifies spacing behavior between text characters" <http://www.w3.org/TR/CSS2/text.html#spacing-props>; not between a character and the right side of the text box.
Safari, Chrome, and Firefox all agree on rendering for this test case. I don't believe there is any remaining compatibility issue.