How to make a simple modal window? [closed]

Question

guys!

Can anyone help to understand how to make a simple modal window? Like to make a fade out background of the web page's content and show a white layer focusing action on it.

I'm going to put on that white focused layer turing number form with ajax query. So if somebody enters the page, first he will see the modal window where he / she will need to fill turing number then by pushing "Submit" button, AJAX will check for correct turing number and show the page if not, it will display an error.

I need just to make modal window without including any libraries like jquery. I want to make a simple modal window with CSS.

Thanks in advance, guys!