Using a percentage margin in CSS but want a minimum margin in pixels?

The true solution here is to use a media query break-point to determine when 33% no longer works for you and should be overridden by the minimum margin in pixels.

/*Margin by percentage:*/
div{
    margin: 0 33% 0 0;
}

/*Margin by pixel:*/
@media screen and ( max-width: 200px ){
    div{
        margin: 0 15px 0 0;
    }
}

In the above code, change the max-width to whatever screen width the 33% right margin no longer works for you.