It`s a great way to let it look like a set background-image height and weight 

出處 http://www.kyrnin.com/about/blfakebgstretchexample.htm

簡單翻譯讓大家可以實用

Faking a Stretched Background Image

假象的底圖縮放

Using the Z-Index Property

I used the z-index and position elements to fake the background on the div to stretch to be wider.

使用 z-index 和position來做

The original image is:  as you can see, much smaller.

原圖是這麼小。

The biggest drawback to this technique is that positioning the text results in extra white space below.

將圖縮放在後面的方法就是使用 position

Learn how to stretch a background image.