CSS中的background属性中的0为什么等效于0% 50%?

background的position其实有两个的即 background-position-xbackground-position-ybackground: url("a.gif") no-repeat 0 = background-position-x = 0；background-position-y = 0background: url("a.gif") no-repeat 0% 50% = background-position-x = 0；backgr