Guide for screen body text

Hope I could have time to build this... I have an idea to build guide body text on website itself. However, I could manage my time to create it. My idea come from this website called twopaperdolls.com
in this website, they have interesting overlay from the background and other layer (which I thought it could be my guide for web) and the background would be other website. I think if the user can type any URL in the box (that i could provide in my website) the background would change into other web, but the guide would stay there. Here is what the website look like: