Can't find a "not equal" css attribute selector

Use the code like this:

div:not([foo=''])
{
    /* CSS Applied to divs having foo value Not nothing (or having a foo value assigned) */
}

:not([foo=''])
{
    background: red;
}

http://jsfiddle.net/gsLvuys0/

One problem with the accepted answer is that it will also select elements that do not have a foo attribute at all. Consider:

<div>No foo</div>
<div foo="">Empty foo</div>
<div foo="x">XXX</div>
<div foo="y">YYY</div>
<div foo="z">ZZZ</div>

div:not([foo='']) will select both the first and second div elements. If you only want div elements that have an attribute foo that is set to an empty string, you should use:

div[foo]:not([foo=''])

If you want all elements with attribute foo that is neither y nor z, you should use:

div[foo]:not([foo='y']):not([foo='z'])