[Windows] Fix crash using complex html content

src/Core/src/Platform/Windows/TextBlockExtensions.cs

@@ -1,7 +1,9 @@
 using System;
+using System.IO;
 using System.Text.RegularExpressions;
+using System.Xml;
 using System.Xml.Linq;
-using Microsoft.UI.Xaml;
+using System.Xml.Resolvers;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Documents;
 
@@ -111,7 +113,7 @@ internal static void UpdateTextHtml(this TextBlock platformControl, ILabel label
 
 			try
 			{
-				var element = XElement.Parse(modifiedText);
+				var element = ParseXhtml(modifiedText);
 				LabelHtmlHelper.ParseText(element, platformControl.Inlines, label);
 			}
 			catch (Exception)
@@ -125,5 +127,33 @@ internal static void UpdateTextPlainText(this TextBlock platformControl, IText l
 		{
 			platformControl.Text = label.Text;
 		}
+
+		static XElement? ParseXhtml(string? html)
+		{
+			if (string.IsNullOrEmpty(html))
+				return null;
+
+			XmlNameTable nt = new NameTable();
+			XmlNamespaceManager nsmgr = new XmlNamespaceManager(nt);
+			var xmlParserContext = new XmlParserContext(null, nsmgr, null, XmlSpace.None);
+			XmlParserContext context = xmlParserContext;
+			context.DocTypeName = "html";
+			context.PublicId = "-//W3C//DTD XHTML 1.0 Strict//EN";
+			context.SystemId = "xhtml1-strict.dtd";
+			XmlParserContext xhtmlContext = context;
+
+			StringReader stringReader = new StringReader(html);
+
+			XmlReaderSettings settings = new XmlReaderSettings
+			{
+				DtdProcessing = DtdProcessing.Parse,
+				ValidationType = ValidationType.DTD,
+				XmlResolver = new XmlPreloadedResolver(XmlKnownDtds.All)
+			};
+
+			XmlReader reader = XmlReader.Create(stringReader, settings, xhtmlContext);
+
+			return XElement.Load(reader);
+		}
 	}
 }

src/Core/tests/DeviceTests/Handlers/Label/LabelHandlerTests.Windows.cs

@@ -1,6 +1,4 @@
-using System;
-using System.Threading.Tasks;
-using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Media;
 
 namespace Microsoft.Maui.DeviceTests

src/Core/tests/DeviceTests/Handlers/Label/LabelHandlerTests.cs

@@ -298,6 +298,23 @@ public async Task LineHeightInitializesCorrectly()
 		}
 #endif
 
+		[Fact(DisplayName = "Html Text Initializes Correctly")]
+		public async Task HtmlTextInitializesCorrectly()
+		{
+			var label = new LabelStub()
+			{
+				TextType = TextType.Html,
+				Text = "<h2><strong>Test1&nbsp;</strong>Test2</h2>"
+			};
+
+			var platformText = await GetValueAsync(label, (handler) =>
+			{
+				return handler.PlatformView.Text;
+			});
+
+			Assert.NotNull(platformText);
+		}
+
 		[Category(TestCategory.Label)]
 		public class LabelTextStyleTests : TextStyleHandlerTests<LabelHandler, LabelStub>
 		{