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

Could you do:

margin-right: 50px;
margin-left: 50px;

Edit: My solution is wrong. The solution posted by Aric TenEyck suggesting using a div with width 100% and then nesting another div using the margins seems more correct.