I looked up the w3specs and the css mailing list, and you already can use 10 Bit colors, by writing somthing like:
rgb(0.1%,0.1%,0.1%) the browser will then decide, if it will round down to 0%
CSS Color Level 4 will also extend rgb() and rgba() to accept a <number>, where an <integer> was used, so that you can write:
rgb(0.25, 0.25, 0.25), which may or may not be rounded to rgb(0, 0, 0)
-- Simon Sapin, on CSS-color Mailing List
For further information read CSS-Color Level 4 - Editor’s Draft, 7 October 2013