例子：
background:url(../images20130624/bg.png) no-repeat -1424px -5px;
官方API
Value: ['background-color'> ||<'background-image'> || <'background-repeat'>|| <'background-attachment'> ||<'background-position'>] | inherit
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: allowed on 'background-position'
Media: visual
Computed value: see individual properties
background属性的值的书写顺序官方并没有强制标准的。
为了可读性，定一个CSS书写规范，规则2.1：background的值的顺序是background-color，background-image，background-repeat，background-attachment，background-position。