Defines how the background image repeats itself across the element's background, starting from the background position.
默认属性
background-repeat: repeat;
The background image will repeat itself both horizontally and vertically.
background-repeat: repeat-x;
The background image will only repeat itself horizontally.
background-repeat: repeat-y;
The background image will only repeat itself vertically.
background-repeat: no-repeat;
The background image will only appear once.