For some reason, Netscape (or Mozilla) doesn't allow for underscores in names of CSS or 'div' or any DHTML 'id' or 'name' attribute after their version 6 browser for Netscape and any Mozilla (that I'm aware of.)
The reason Netscape & Mozilla don't allow "_" is that it's not legal CSS. See the appropriate dull bit from the spec. Not a bug :-)
I lied. Underscores have been allowed since the 1998 errata.