Avalonia/samples/Sandbox/MainWindow.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
        x:Class="Sandbox.MainWindow">
  <Border Background="LightGray" HorizontalAlignment="Center" VerticalAlignment="Center">
    <StackPanel>
      <RichTextBlock x:Name="txtBlock" IsTextSelectionEnabled="True" Margin="10" TextWrapping="Wrap">
        This <Span FontWeight="Bold">is</Span> a
        <Span Background="Silver" Foreground="Maroon">TextBlock</Span>
        with <Span TextDecorations="Underline">several</Span>
        <Span FontStyle="Italic">Span</Span> elements,
        <Span Foreground="Blue">
          using a <Bold>variety</Bold> of <Italic>styles</Italic>
        </Span>.
      </RichTextBlock>
      <TextBox Text="{Binding #txtBlock.SelectedText}"/>
      <TextBlock Text="{Binding #txtBlock.Text}"/>
    </StackPanel>
  </Border>
</Window>