Same XAML Silverlight and WPF – different result

I did not know the following would produce different results in WPF and Silverlight

<Grid>
        <TextBlock Text=”{Binding RelativeSource={RelativeSource Self}, Path=Tag}” Tag=”hello”/>
    </Grid>

In WPF, you see “Hello”. In silverlight you dont see anything

The following XAML produces same result in WPF and Silverlight

<Grid>
        <TextBlock Tag=”hello” Text=”{Binding RelativeSource={RelativeSource Self}, Path=Tag}” />
    </Grid>