a div above another?

i have some php files with html code in it and from my php file i have:

 require "popup.html";
 require "main.html";

then i use css to hide the div in popup.html.

when i click a link coupled to jquery to display the popup it will be above all divs in main.html except the on开发者_如何学Pythones that i have rendered in with ajax.

how can i make the popup div above all these ajaxed stuff?

z-index