This isn't possible without peppering your HTML with a stack of extraneous class selectors, sadly.
I'd recommend designing your site so that your entirely valid CSS works for people using modern browsers, and that it's still usable in the IE6, albeit visually not quite right. You just have to find the right balance between getting your site up to standard and bending over backwards for users who won't upgrade. It's a broken browser, treat it as such.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…