To solve your problem, you can add these style attributes to your btn1 and btn2 anchor buttons:
border-top-left-radius: 41px; border-top-right-radius: 41px; border-bottom-left-radius: 41px; border-bottom-right-radius: 41px;
I tried with IE10 and it works.
Example for btn1:
<a id="btn1" href="#" style="border-top-left-radius: 41px; border-top-right-radius: 41px; border-bottom-right-radius: 41px; border-bottom-left-radius: 41px;"/>
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