逐步改变 max-width, 从 "none" 到 "600px":

This DIV element does not have a pre-defined width.

Try resizing the browser window to see that width of this DIV element will grow/shrink along width the resizing, but it will never exceed 600 pixels in width.

max-width 属性支持动画效果。

注意: 目前 max-height 属性在 Chrome 浏览器下不会改变 gradually（灰阶）, 在 Firefox 都无法起作用。

注意：Internet Explorer 9 及更早 IE 版本不支持 CSS 动画。