Selecting children elements in IE

children.css

/* equivalent to li:nth-child(1) */
#mainNav ul li:first-child a {
    background: #1b77af;
    width: 236px;
    text-align: center;
    margin-right: 10px;
}
/* equivalent to li:nth-child(2) */
#mainNav ul li:first-child + li a {
    background: #33a0ff;
}
/* equivalent to li:nth-child(3) */
#mainNav ul li:first-child + li + li a {
    background: #de7803;
}​

/* equivalent to li:nth-child(4) */
#mainNav ul li:first-child + li + li + li a {
    background: #d94e4e;
}​