jquery -Get CSS properties values for a not yet applied class

In order to read a CSS property value from a nonexistent element, you need to dynamically insert that element (as hidden) to the DOM, read the property and finally remove it:

var getCSS = function (prop, fromClass) {

    var $inspector = $("<div>").css('display', 'none').addClass(fromClass);
    $("body").append($inspector); // add to DOM, in order to read the CSS property
    try {
        return $inspector.css(prop);
    } finally {
        $inspector.remove(); // and remove from DOM
    }
};

jsFiddle here

Great answer by Jose. I modified it to help with more complex css selectors.

var getCSS2 = function (prop, fromClass, $sibling) {

    var $inspector = $("<div>").css('display', 'none').addClass(fromClass);
    if($sibling != null){
        $sibling.after($inspector); //append after sibling in order to have exact 
    } else {
        $("body").append($inspector); // add to DOM, in order to read the CSS property
    }
    try {
        return $inspector.css(prop);
    } finally {
        $inspector.remove(); // and remove from DOM
    }
};

JSFiddle