How can I include an OnPlatform in a style resource?
Define every property as OnPlatform
<OnPlatform x:Key="MyFontSize" x:TypeArguments="x:Double" iOS="14" Android="14" WinPhone="14" />
and refer it in yout button style like
<Style x:Key="MyButtonStyle" TargetType="{x:Type Button}">
<Setter Property="FontSize" Value="{StaticResource MyFontSize}" />
</Style>
With the new Xamarin OnPlatform specification is possible to define the platform styles directly in the style tag:
<Style x:Key="buttonStyle" TargetType="Button">
<Setter Property="FontSize">
<OnPlatform x:TypeArguments="x:Double">
<On Platform="Android">40</On>
<On Platform="iOS">10</On>
</OnPlatform>
</Setter>
</Style>