IE 5/5.5 copes with the underscores ok even though they're not valid but other browsers including IE4/N6 don't.
W3 : Cascading Style Sheets, level 2 CSS2 Specification
In CSS2, identifiers (including element names, classes, and IDs in selectors [p. 53] ) can contain only the characters [A-Za-z0-9] and ISO 10646 charac-ters 161 and higher, plus the hyphen (-); they cannot start with a hyphen or a digit. They can also contain escaped characters and any ISO 10646 charac-ter as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B\&W\?" or "B\26 W\3F".