This is a demo of how CSS can be used to create a popup. The old ways of creating popups by opening a new browser window is no longer a good solution since browsers block popups.
This page is set to open the popup when it's loaded (onload), and by clicking the link below you can also open the popup (onclick), giving you a couple different options to trigger the CSS popup.
Check out the tutorial here to find out what's going on and to download the files you need to create your own CSS popup window.