How can I do width = 100% - 100px in CSS?

Question

In CSS, how can I do something like this:

width: 100% - 100px;

I guess this is fairly simple but it is a bit hard to find examples showing

Answer 1

The short answer is you DON'T do this in CSS. Internet Explorer has support for something called CSS Expressions, but this isn't standard and is definitely not supported by other browsers like FireFox for instance.

You'd be better off doing this in JavaScript.