How to perform arithmetic operations in CSS?

It already exists; You can use the CSS3 calc() notation:

div {
    background: olive;
    height: 200px;
    width: 200px;
}

div > div {
    background: azure;
    height: calc(100% - 10px);
    width: 100px;
}

http://jsfiddle.net/NejMF/

^{Note: It’s only supported in modern browsers (IE9+) and has only recently been adopted by mobile browsers.}