Following is the example for creating a popup using CSS.
Step 1: Create an empty web application.
Step 2: Create a page in that web application.
Step 3: Add a div in that page as below.
Step 4: Add a anchor tag to open the popup. It can be done even with server side controls also.
Step 5: Add styles for the popup to open.
Step 8: Run the application. Click on the "Click here to open the popup link". You can see the popup opened.