BrowserBehavior.cs

using System.Windows;
using System.Windows.Controls;

namespace TVRename
{
    public static class BrowserBehavior
    {
        public static readonly DependencyProperty HtmlProperty = DependencyProperty.RegisterAttached(
            "Html",
            typeof(string),
            typeof(BrowserBehavior),
            new FrameworkPropertyMetadata(OnHtmlChanged));

        [AttachedPropertyBrowsableForType(typeof(WebBrowser))]
        public static string GetHtml(this WebBrowser d)
        {
            return (string)d.GetValue(HtmlProperty);
        }

        public static void SetHtml(this WebBrowser d, string value)
        {
            d.SetValue(HtmlProperty, value);
        }

        static void OnHtmlChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            WebBrowser wb = d as WebBrowser;
            wb?.NavigateToString(e.NewValue as string??string.Empty);
        }
    }
}