How to manage CSS layout dependencies?

How to manage CSS layout dependencies?

The (probably) best solution is to use a CSS preprocessor such as SASS. You can do a really simple variable assignment and calculation such as:

$leftMenuWidth: 150px;

#left_menu {
    width: $leftMenuWidth;
    float: left;
}

#content {
    margin-left: $leftMenuWidth + 20;   /* must be kept as left_menu.width+20 */
    border: 1px solid white;
}