使用calc（）中先前定义的样式的CSS

Question

如何在我的calc()中使用CSS（3）中以前定义的样式？我想知道如何，因为我可以使用类似width: calc(80% - calc(padding / 2))的内容，但我不确定如何直接在CSS中获取padding。

#main {
    padding: 10px;
    /* This is how we might do it - remember, we want our width to be 100% - 20px. */
    /* See this.padding? */
    width: calc(80% - calc(this.padding * 2));
    /* Of course, it could be something like self.padding: */
    width: calc(80% - calc(self.padding * 2));
    /* Or even just (and best of all): */
    width: calc(80% + calc(padding * 2));
}

/* What would be even more awesome is if we could get properties from other styles: */

#demo1 {
    font-family: Monaco;
}

#demo2 {
    font-family: #demo1;
}

/* Of course, we can do this if #demo2 is inside of #demo1: */

#demo1 {
    font-family: Monaco;
}

#demo2 {
    font-family: inherit;
}

/* But we want to do this without having it use inherit. */

知道怎么做这些吗？我不想使用SASS或var，因为它仅在Firefox中受支持。