Content of the BUTTON element moves when clicked

Hi,

When clicking the HTML BUTTON element (at least in Opera 9.51 and Opera 9.27 on Win XP 32) the content of the element moves down and right by 1px. I tried to figure out how this happens by using the Dragonfly Alpha 2 developer tool but it didn't help.

While this sounds only a minor issue it can give you a nice headache when using the Sliding Doors CSS technique with the BUTTON elements: http://www.filamentgroup.com/lab/styling_the_button_element_with_sliding_doors/ (this also where one of the commenters posted the issue).

The same "movement" happens with IE6 and IE7 but can be fixed with CSS.

Any ideas or similar experiences?

Thanks!