How do I make a transparent background with elements behind it, if possible?

I'm sort of interested in making a website (currently learning) and I have seen some interesting effect where you scroll the content of the page, but some elements remain static.

Kind of an effect where you scroll the webpage and one part has transparent background and you see everything behind it.

You can see such effect when you scroll to the very bottom of http://www.noip.com/.

Is it possible to do using only HTML and CSS? I assume I have to set the appropriate material background to transparent (which I don't know), and set some static elements behind it.

Speaking of my testing website, here's the idea so you can understand easier. It looks like this;

http://ift.tt/1SjanuA

And I think setting that full-sized landscape picture as a static background behind all elements, and setting the background of that line to transparent would do the thing, would it?

Any suggestion appreciated.