Hello Philippe,
I have tested the reported issue but I could not reproduce it with the following code:
Attached is btnIE9.gif showing the result in IE9 from the code above. I do not see any problem from our side. In order to help you further we will need a screenshot with the problem, and a small project where the issue is reproduced, because I think it could be caused by some specific settings in your application - such as CSS that overrides the decorated buttons styles. Try to remove an external stylesheet with your styles linked to the page and check if it is fixed. If it is fixed, then the problem should come form some custom CSS styles applied by you.
Best wishes,
Bozhidar
the Telerik team
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