Watch the demo at bl.ocks.org.
Source and comments on gist.github.com.
Thought about that but then the snow would not fall straight down as several spans of NBSPs could fit on the width of the window. :\
If you're all cool with this effect only working properly in a narrow viewport, then I believe we can replace the \n by a \t or even U+0009 character which saves 1 to 2 extra bytes