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

Question

I want that my background image stretch and scale depending on the browser viewport size.

I\'ve seen some questions on Stack Overflow that do the job, like

Answer 1

Do you want to achieve this just using one image? Because you can actually make somewhat similar to a stretching background using two images. PNG images for instance.

I've done this before, and it's not that hard. Besides, I think stretching would just harm the quality of the background. And if you add a huge image it would slow down slow computers and browsers.