A very obscure bug

I found that bit of javascript that creates snow falling down the screen. Actually I found two versions, one had the snowflakes as tiny GIFs, the other as ASCII stars. Too late for this Xmas, but next year I'll be annoying people with them (they eat CPU time).

Except that the ASCII version didn't work in Firefox. After steadily cutting out the usual suspects - other javascript for example - the culprit turned out bizarrely to be the URL part of the DTD:

!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd"

So there you are, my first trivia for the year.