WPF Storyboard xaml code----------------------MainWindow.xaml

    <Grid>
        <StackPanel Height="289" HorizontalAlignment="Left" Margin="10,10,0,0" Name="stackPanel1" VerticalAlignment="Top" Width="464" />
        <Rectangle Name="myrectangle" Width="100" Height="100" Fill="Bisque">
            <Rectangle.Triggers>
                <EventTrigger RoutedEvent="Rectangle.Loaded">
                    <BeginStoryboard>
                        <Storyboard>
                            <DoubleAnimation Storyboard.TargetName="myrectangle" Storyboard.TargetProperty="Opacity" From="1.0" To="0.0" Duration="0:0:5" AutoReverse="True" RepeatBehavior="Forever"></DoubleAnimation>
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </Rectangle.Triggers>
           
        </Rectangle>
    </Grid>

OUTPUT: