"." in class selectors where class names have special characters

Hi,

http://www.w3.org/TR/CSS2

The above document doesn't explain what characters are allowed when
"." is used as the class selectors. For example,

<div class="100.100"></div>

Selecting the above element by div.100.100 seems not to work.

Do I miss anything in the document? Or this is discussed in some other
document that I'm not aware of.

-- 
Regards,
Peng