Spot the difference - Link vs button

[sergiozambrano]

What's the attribute (probably browser default) that makes the background-image to overflow with the text or stay with the box?

Time ago I wrote about a technique for creating a css transparent button with only one image, no sprites: http://socialblogsitewebdesign.com/s...t-button-ever/

Basically, in that example a submit input tag "hangs" OUTSIDE the container's width but using the following tags won't work:

<a><span>Click me</span></a>

What is the difference between a label/input combination and a a/span (even a/div) combination that makes the background of the inner element not extending outside the container along with the text?

Thanks