Do you mean according to the spec? Seems to be.

[letter-spacing](http://www.w3.org/TR/CSS21/text.html#propdef-letter-spacing) takes a [length](http://www.w3.org/TR/CSS21/syndata.html#value-def-length). A length is a [number](http://www.w3.org/TR/CSS21/syndata.html#value-def-length) ("zero or more digits followed by a dot (.) followed by one or more digits") followed by a unit identifier.