Main.axaml

<UserControl
  xmlns="https://github.com/avaloniaui"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:i="clr-namespace:Avalonia.Xaml.Interactivity;assembly=Avalonia.Xaml.Interactivity"
  xmlns:ia="clr-namespace:Avalonia.Xaml.Interactions.Core;assembly=Avalonia.Xaml.Interactions"
  MaxWidth="600">
    <UserControl.Styles>
      <Styles>
        <Style Selector="Border.animated">
          <Style.Animations>
            <Animation Duration="0:0:3"
                       IterationCount="Infinite"
                       PlaybackDirection="Normal">
              <KeyFrame Cue="0%">
                <Setter Property="OpacityMask">
                  <ConicGradientBrush Center="50%,50%" Angle="-70">
                    <GradientStop Offset="0" Color="Black"/>
                    <GradientStop Offset="0.001" Color="Transparent"/>
                    <GradientStop Offset="1" Color="Transparent"/>
                  </ConicGradientBrush>
                </Setter>
                <Setter Property="Opacity" Value="1" /> <!-- hack as OpacityMask doesn't invalidate visual -->
              </KeyFrame>
              <KeyFrame Cue="90%">
                <Setter Property="OpacityMask">
                  <ConicGradientBrush Center="50%,50%" Angle="-70">
                    <GradientStop Offset="0" Color="Black"/>
                    <GradientStop Offset="1" Color="Transparent"/>
                  </ConicGradientBrush>
                </Setter>
                <Setter Property="Opacity" Value="0.9998" /> <!-- hack as OpacityMask doesn't invalidate visual -->
              </KeyFrame>
              <KeyFrame Cue="100%">
                <Setter Property="OpacityMask">
                  <ConicGradientBrush Center="50%,50%" Angle="-70">
                    <GradientStop Offset="0" Color="Black"/>
                    <GradientStop Offset="0.999" Color="Black"/>
                    <GradientStop Offset="1" Color="Transparent"/>
                  </ConicGradientBrush>
                </Setter>
                <Setter Property="Opacity" Value="0.9999" /> <!-- hack as OpacityMask doesn't invalidate visual -->
              </KeyFrame>
            </Animation>
          </Style.Animations>
        </Style>
      </Styles>
    </UserControl.Styles>
    <Grid>
      <Border Classes="animated" BorderThickness="3" BorderBrush="Blue" Width="100" Height="40" CornerRadius="5">
      </Border>
    </Grid>
</UserControl>