MainWindow.xaml

<Window x:Class="toucca.MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:local="clr-namespace:toucca"
      xmlns:wv2="clr-namespace:Microsoft.Web.WebView2.Wpf;assembly=Microsoft.Web.WebView2.Wpf"
      mc:Ignorable="d"
      Title="Toucca"
      Height="800"
      Width="800"
      WindowStyle="None"
      AllowsTransparency="True"
      Background="Transparent"
      >

        <Border Background="#01000000">
        <wv2:WebView2 Name="webView"  HorizontalAlignment="Stretch"
            VerticalAlignment="Stretch" Source="http://localhost:25730" Initialized="webView_Initialized" DefaultBackgroundColor="Transparent"/>
        </Border>

</Window>