One of the projects which I am working uses CSS "attribute" selector [att]
which is not supported by ie6: Support for CSS selectors in IE6 (look for text "Attribute Selectors")
Is there any workaround/hack which is of course valid html/css to overcome this problem?
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.