contain values for the
background-size property makes it easy to create background images that cover the complete browser window, but for a very long time web developers didn’t have control over much else: smaller tiled images repeated infinitely in backgrounds, with no control over how they started or finished.
By default, background images tile both vertically and horizontally; CSS provided the opportunity to constrain this to one axis with
background-repeat: repeat-x or
background-repeat: repeat-y, but that’s as far as it went. What designers needed was “flexbox for backgrounds”: the ability to evenly distribute tiled images.
With the release of Firefox 49, that feature is now available in every major browser, in the form of