Adding:

Intellisense - I've only added TextBlock but we can easily add any of the XAML elements
Error highlighting works - Only issue is that the XAML parser doesn't seem to be that accurate with where the errors actually are, so perhaps a bit of refinement to be done but at least the user sees the marking in the right spot

src/Uno.Playground.Shared/Samples/Playground.xaml.cs

@@ -34,6 +34,10 @@
 using Uno.Logging;
 using Uno.UI.Demo.Behaviors;
 using Uno.UI.Toolkit;
+using Monaco.Languages;
+using Monaco.Editor;
+using Monaco;
+using System.Threading;
 
 namespace Uno.UI.Demo.Samples
 {
@@ -63,6 +67,7 @@ public Playground()
 
 			xamlText.PropertyChanged += OnPropertyChanged;
 			xamlText.Loaded += OnEditorLoaded;
+			xamlText.Loading += OnEditorLoading;
 
 			jsonDataContext.TextChanged += OnDataContextTextChanged;
 
@@ -84,6 +89,7 @@ public Playground()
 
 
 		}
+
 		private void OnInputPaneShowing(InputPane sender, InputPaneVisibilityEventArgs args)
 		{
 			VisibleBoundsPadding.SetPaddingMask(TabsPane, VisibleBoundsPadding.PaddingMask.Top);
@@ -263,6 +269,11 @@ private void OnExpandoChanged(object sender, PropertyChangedEventArgs e)
 			jsonDataContext.Text = data;
 		}
 
+		private async void OnEditorLoading(object sender, RoutedEventArgs e)
+		{
+			await xamlText.Languages.RegisterCompletionItemProviderAsync("xml", new XamlLanguageProvider());
+		}
+
 		private async void OnEditorLoaded(object sender, RoutedEventArgs e)
 		{
 			xamlText.CodeLanguage = "xml";
@@ -390,17 +401,19 @@ private async Task Update()
 			}
 		}
 
+		/// When parsing XAML we need to prepend a Grid element (see GetXamlInput) which adds lines to the start
+		/// of the XAML, causing any errors to be off by that many lines
+		private int GetXamlPrefixLineCount => 4;
 		private string GetXamlInput()
 		{
 			Console.WriteLine("Current text: " + xamlText.Text);
-			return string.Concat(
-				@"<Grid
-					xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
-					xmlns:x= ""http://schemas.microsoft.com/winfx/2006/xaml""
-					xmlns:behaviors=""using:Uno.UI.Demo.Behaviors"">",
-				xamlText.Text,
-				"\n</Grid>"
-			);
+			return 
+$@"<Grid
+	xmlns=""http://schemas.microsoft.com/winfx/2006/xaml/presentation""
+	xmlns:x= ""http://schemas.microsoft.com/winfx/2006/xaml""
+	xmlns:behaviors=""using:Uno.UI.Demo.Behaviors"">
+{xamlText.Text}
+</Grid>";
 		}
 
 		private static readonly string _appName =
@@ -442,7 +455,7 @@ private async void ShowHint()
 
 		private void SetLink(string id, bool clipboard = false)
 		{
-			var url = new Uri($"{PlaygroundUrl}#{Uri.EscapeUriString(id)}");
+			var url = new System.Uri($"{PlaygroundUrl}#{System.Uri.EscapeUriString(id)}");
 			if (clipboard)
 			{
 				var clipboardData = new DataPackage();
@@ -480,7 +493,7 @@ private async Task LoadSample(string id)
 					xamlText.Text = "<!-- loading... -->";
 					jsonDataContext.Text = "/* loading... */";
 
-					var response = await httpClient.GetAsync($"{BaseApiUrl}/{Uri.EscapeUriString(id)}");
+					var response = await httpClient.GetAsync($"{BaseApiUrl}/{System.Uri.EscapeUriString(id)}");
 					var responsePayload = await response.Content.ReadAsStringAsync();
 					var json = JObject.Parse(responsePayload);
 
@@ -561,21 +574,49 @@ private void Update_OnTapped(object sender, RoutedEventArgs e)
 			LaunchUpdate();
 		}
 
-		private void ClearError()
+		private async void ClearError()
 		{
 			_currentError = null;
-			errorBorder.Visibility = Visibility.Collapsed;
-			errorText.Text = "No error";
+			if (errorBorder.Visibility == Visibility.Visible)
+			{
+				errorBorder.Visibility = Visibility.Collapsed;
+				errorText.Text = "No error";
+				//xamlText.Markers.Clear();
+				await xamlText.SetModelMarkersAsync("CodeEditor", Array.Empty<IMarkerData>());
+			}
 		}
 
-		private void ShowError(Exception error)
+		private async void ShowError(Exception error)
 		{
 			_currentError = error;
 			if (error != null && !_isLoadingSample)
 			{
 				errorText.Text = error.Message;
+				if(errorBorder.Visibility == Visibility.Visible)
+				{
+					// Only clear the errors if the error bubble is already showing, in which
+					// case we need to update the errors
+					await xamlText.SetModelMarkersAsync("CodeEditor", Array.Empty<IMarkerData>());
+				}
 				errorBorder.Visibility = Visibility.Visible;
+
+				if (error is System.Xml.XmlException xamlError)
+				{
+					xamlText.Markers.Add(
+						new MarkerData()
+						{
+							Code = "0000",
+							Message = error.Message,
+							Severity = MarkerSeverity.Error,
+							Source = "Origin",
+							StartLineNumber = (uint)(xamlError.LineNumber- GetXamlPrefixLineCount),
+							StartColumn = (uint)xamlError.LinePosition,
+							EndLineNumber = (uint)(xamlError.LineNumber- GetXamlPrefixLineCount),
+							EndColumn = (uint)xamlError.LinePosition+5
+						});
+				}
 			}
+
 		}
 
 		private void CopyError(object sender, RoutedEventArgs e)
@@ -676,7 +717,7 @@ private void RestoreCodePaneSize(Size oldSize, Size newSize)
 
 		private async void LogoClicked(object sender, RoutedEventArgs e)
 		{
-			await Launcher.LaunchUriAsync(new Uri("http://platform.uno/"));
+			await Launcher.LaunchUriAsync(new System.Uri("http://platform.uno/"));
 		}
 
 		private void ShowXaml(object sender, RoutedEventArgs e) => SelectTab("XAML");
@@ -753,4 +794,62 @@ private void SelectTab(string pane)
 			}
 		}
 	}
+
+
+	public class XamlLanguageProvider : CompletionItemProvider
+	{
+		public string[] TriggerCharacters => new string[] { "<" };
+
+		public IAsyncOperation<CompletionList> ProvideCompletionItemsAsync(IModel document, Position position, CompletionContext context)
+		{
+			return AsyncInfo.Run(async delegate (CancellationToken cancelationToken)
+			{
+				var textUntilPosition = await document.GetValueInRangeAsync(new Range(1, 1, position.LineNumber, position.Column));
+
+				if (textUntilPosition.EndsWith("boo"))
+				{
+					return new CompletionList()
+					{
+						Suggestions = new[]
+						{
+							new CompletionItem("booyah", "booyah", CompletionItemKind.Folder),
+							new CompletionItem("booboo", "booboo", CompletionItemKind.File),
+						}
+					};
+				}
+				else if (context.TriggerKind == CompletionTriggerKind.TriggerCharacter)
+				{
+					return new CompletionList()
+					{
+						Suggestions = new[]
+						{
+							new CompletionItem("TextBlock", "TextBlock>\n\t$0\n</TextBlock", CompletionItemKind.Snippet)
+						{
+							InsertTextRules = CompletionItemInsertTextRule.InsertAsSnippet
+						},
+						}
+					};
+				}
+
+				return new CompletionList()
+				{
+					Suggestions = new[]
+					{
+						new CompletionItem("foreach", "foreach (var ${2:element} in ${1:array}) {\n\t$0\n}", CompletionItemKind.Snippet)
+						{
+							InsertTextRules = CompletionItemInsertTextRule.InsertAsSnippet
+						}
+					}
+				};
+			});
+		}
+
+		public IAsyncOperation<CompletionItem> ResolveCompletionItemAsync(IModel model, Position position, CompletionItem item)
+		{
+			return AsyncInfo.Run(delegate (CancellationToken cancelationToken)
+			{
+				return Task.FromResult(item); // throw new NotImplementedException();
+			});
+		}
+	}
 }