支持多 background-image

[temper357]

CSS background 属性支持配置多个 background-image https://www.w3.org/TR/css-backgrounds-3/#layering

background-image: url(flower.png), url(ball.png), url(grass.png);
background-position: center center, 20% 80%, top left, bottom right;
background-origin: border-box, content-box;
background-repeat: no-repeat;