You can just add the following code and the browser will force to IE8 mode (which renders correctly):
Add this to the top of your HTML:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Add this to the head section of your HTML
<meta http-equiv="X-UA-Compatible" content="IE=8" />
The button will then render correctly. See a live example here.