(Responsive HTML/CSS) How to set the height of a div box to the device screen height

I am going for the admittedly worn-out design where you have a static background image, and once the user scrolls down the content comes over the image.

I have my background image in place and it is responsive:

html {        
background: url(Images/Frontpage.jpg) no-repeat center center fixed;
}

and I now want to add an emptycontainer spanning the height of the user's screen along the screen, then add my content below that so the user scrolls down and sees the content.

This webpage pretty much does this, but without using a static background, just making every div the height of the device height.

I foolishly tried using percentages and the em value to no avail.

Any and all feedback is greatly appreciated!