/* [1] for ie6 */
div#hoge {
color:#f00;
}

/* [2] for fx and safari and opera and... */
html[xmlns] div#hoge {
color:#0f0;
}

/* [3] for ie7 */
*:first-child+html div#hoge {
color:#00f;
}

[2]を効かすにはhtmlタグに要xmlns属性。