This is my asp:button code which is not rendering font awesome\'s icon but instead shows the HTML as it is:
You can also try this solution -
<span style="position:relative;">
<i class="fa fa-hand-o-down"></i>
<asp:Button ID="btnCatMoveDown" CssClass="movedown" CausesValidation="false" Text="" CommandName="categorymovedown" CommandArgument='<%#Eval("SomeId")%>' runat="server"></asp:Button>
</span>
<style>
.movedown {
position:absolute;
opacity:0;
top:0;
left:0;
width:100%;
height:100%;
}
</style>