Say if I have a class like this:

.middle {
width: 60%;
color: red;
}

But I want to apply if to a DIV but I want the width to be 100%

Can I do that (and if so how) or do I have to create a new class?

Thanks.

BTW A div will be 100% if you don't apply any CSS rules to it.

set a new rule specifically for div elements with class "middle" that will override the properties in the global class

div.middle {
   width:100%;
}

so a div with class "middle" will have all the rules in "middle" applied apart from the width.
ensure the div rule is after the global in the stylesheet hierarchy

The thing is, they are both DIVS and then I have other styles like .middle h1, .middle a:link etc

If I created a new class and apply it to the DIV it won't get all the styles from .middle h1, .middle a:link etc and I will have to create div.middle h1, div.middle a:link etc

Is that correct?

elements can have multiple classes defined in a space delimited list:
eg:

<div class="middle mid_full_width">

.mid_full_width{
   width:100%;
}

then that div will then have rules for "middle" and all the "middle" cascading rules plus the overrides from "mid_full_width"

Thanks that is perfect.