Your question is already answered but I recommend you Twitter Bootstrap.
It is a HTML/CSS framework which allows you make great buttons (among others) easily.
This buttons are "crossbrowser" which means they are compatible with Internet Explorer.
With only one class - btn
- you can stylize a <button>
, <input type="button">
or <a>
.
This :
<button class="btn"></button>
Gives that:

You also can customize your button quickly with an optional class :
btn-primary
, btn-info
, btn-success
, btn-warning
, btn-danger
and btn-inverse
.
This classes transform your button like that :

Then you can choose a size with the classes btn-large
, btn-small
and btn-mini
.
Finally, you can add an icon (the list is here) in your button.
This:
<button class="btn btn-success">
<i class="icon-shopping-cart icon-white"></i>
<span>Checkout</span>
</button>
Gives that:

Have fun with your new buttons. :)