Stretch and scale a CSS image in the background - with CSS only

Question

I want my background image to enlarge and shrink according to the size of the browser viewport.

On Stack Overflow, I've come across some queries that are useful, such as Stretch and scale CSS background. It functions perfectly, but I want to insert the image using the backdrop rather than the img tag.

In that one, an img tag is used, and we then pay homage to the img tag using CSS.

width:100%; height:100%;

It is true that enlarging a background image in CSS 3 will function fairly well, although that query is a little bit out of date. This first example is one that I tried, but it didn't work out for me.

Is there a decent way to use the background-image declaration for this?