In a web LOB web based SaaS product we are developing that we explicitly not support IE 6, only IE7/8, FF 3, Chrome, Opera, WebKit etc... which allow px resize as standard, is it still important to use em rather than px? Its a lot more work to consider the compute font size (size em are computed from their inheritance chain) especially when nesting html reuse components where a font-size has already been specified in an outer container.