Pass the value as a string to the .css
method
div.css('height','60%');
if it is depended on the window width you will have to make a test
if (screen < 500){ /*500 is an example here*/
div.css('height','60%');
}
Update of answer due to the comments
The OP wanted to keep a fixed ration of width/height on some element.
The solution was to resize the div as the browser width changed
in terms of the OP's code the update is
$(function() {
var div = $('.YB_full_content_img_header');
$(window).resize(function(){
var width = div.width();
div.css('height', width * 60/100);
}).trigger('resize');
});
but an alternate option is to use the CSS padding trick (Maintain the aspect ratio of a div with CSS)