It's not often that jQuery UI developers find themselves worrying about reading and setting the
z-index CSS property. That's because it's not all that relevant - it controls the ordering of elements on the stack. For instance, items with higher values will move toward the front of the screen when overlapped with another element with a lower
z-index value. When you're designing your web page, or some part of your web application, you're going to statically align and layout the various components at design time. Responsive design let's our elements respond to varying client browser environments - think of different screen sizes and the user changing the orientation or the dimensions of the browser window. Beyond this, unless the user is directly rearranging elements on the page, we're not too worried about the ordering of overlapping elements. But users do rearrange jQuery UI elements.