The background-size CSS property specifies the size of the background images. The size of the image can be fully constrained or only partially in order to preserve its intrinsic ratio.

[Webplatform](docs.webplatform.org/wiki/css/properties/background-size "Webplatform")
[Mozilla docs](https://developer.mozilla.org/en-US/docs/Web/CSS/background-size "Mozilla")
[Caniuse](http://caniuse.com/#feat=background-img-opts "Caniuse")