diff --git a/samples/Sandbox/MainWindow.axaml b/samples/Sandbox/MainWindow.axaml
index 6929f192c7..03f80d8fc7 100644
--- a/samples/Sandbox/MainWindow.axaml
+++ b/samples/Sandbox/MainWindow.axaml
@@ -1,4 +1,13 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
+  <RichTextBlock 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>
 </Window>
diff --git a/src/Markup/Avalonia.Markup.Xaml/IAddChild.cs b/src/Avalonia.Base/Metadata/IAddChild.cs
similarity index 61%
rename from src/Markup/Avalonia.Markup.Xaml/IAddChild.cs
rename to src/Avalonia.Base/Metadata/IAddChild.cs
index 993eb6142d..8ef02912a6 100644
--- a/src/Markup/Avalonia.Markup.Xaml/IAddChild.cs
+++ b/src/Avalonia.Base/Metadata/IAddChild.cs
@@ -1,11 +1,11 @@
-namespace Avalonia.Markup.Xaml
+namespace Avalonia.Metadata
 {
     public interface IAddChild
     {
         void AddChild(object child);
     }
 
-    public interface IAddChild<T> : IAddChild
+    public interface IAddChild<T>
     {
         void AddChild(T child);
     }
diff --git a/src/Avalonia.Controls/Documents/Inline.cs b/src/Avalonia.Controls/Documents/Inline.cs
index b400625903..47581e87f1 100644
--- a/src/Avalonia.Controls/Documents/Inline.cs
+++ b/src/Avalonia.Controls/Documents/Inline.cs
@@ -50,8 +50,38 @@ namespace Avalonia.Controls.Documents
 
         protected TextRunProperties CreateTextRunProperties()
         {
-            return new GenericTextRunProperties(new Typeface(FontFamily, FontStyle, FontWeight), FontSize,
-                TextDecorations, Foreground, Background, BaselineAlignment);
+            var textDecorations = TextDecorations;
+            var background = Background;
+
+            if(Parent is Inline inline)
+            {
+                if(textDecorations == null)
+                {
+                    textDecorations = inline.TextDecorations;
+                }
+
+                if(background == null)
+                {
+                    background = inline.Background;
+                }
+            }
+            
+            var fontStyle = FontStyle;
+
+            if(Parent is Italic)
+            {
+                fontStyle = FontStyle.Italic;
+            }
+
+            var fontWeight = FontWeight;
+
+            if(Parent is Bold)
+            {
+                fontWeight = FontWeight.Bold;
+            }
+
+            return new GenericTextRunProperties(new Typeface(FontFamily, fontStyle, fontWeight), FontSize,
+                textDecorations, Foreground, background, BaselineAlignment);
         }
         
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
diff --git a/src/Avalonia.Controls/Documents/InlineCollection.cs b/src/Avalonia.Controls/Documents/InlineCollection.cs
index dc688fc359..3d7a4b58d1 100644
--- a/src/Avalonia.Controls/Documents/InlineCollection.cs
+++ b/src/Avalonia.Controls/Documents/InlineCollection.cs
@@ -14,7 +14,6 @@ namespace Avalonia.Controls.Documents
     {
         private ILogical? _parent;
         private IInlineHost? _inlineHost;
-        private string? _text = string.Empty;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="InlineCollection"/> class.
@@ -22,7 +21,7 @@ namespace Avalonia.Controls.Documents
         public InlineCollection()
         {
             ResetBehavior = ResetBehavior.Remove;
-            
+
             this.ForEachItem(
                 x =>
                 {
@@ -36,16 +35,16 @@ namespace Avalonia.Controls.Documents
                     x.InlineHost = InlineHost;
                     Invalidate();
                 },
-                () => throw new NotSupportedException());         
+                () => throw new NotSupportedException());
         }
 
-        internal ILogical? Parent 
+        internal ILogical? Parent
         {
             get => _parent;
             set
             {
                 _parent = value;
-              
+
                 OnParentChanged(value);
             }
         }
@@ -61,8 +60,6 @@ namespace Avalonia.Controls.Documents
             }
         }
 
-        public bool HasComplexContent => Count > 0;
-
         /// <summary>
         /// Gets or adds the text held by the inlines collection.
         /// <remarks>
@@ -73,11 +70,6 @@ namespace Avalonia.Controls.Documents
         {
             get
             {
-                if (!HasComplexContent)
-                {
-                    return _text;
-                }
-
                 var builder = new StringBuilder();
 
                 foreach (var inline in this)
@@ -87,17 +79,7 @@ namespace Avalonia.Controls.Documents
 
                 return builder.ToString();
             }
-            set
-            {
-                if (HasComplexContent)
-                {
-                    Add(new Run(value));
-                }
-                else
-                {
-                    _text = value;
-                }
-            }
+
         }
 
         /// <summary>
@@ -110,36 +92,12 @@ namespace Avalonia.Controls.Documents
         /// <param name="text"></param>
         public void Add(string text)
         {
-            if (HasComplexContent)
-            {
-                Add(new Run(text));
-            }
-            else
-            {
-                _text = text;
-            }
+            Add(new Run(text));
         }
 
         public void Add(IControl child)
         {
-            var implicitRun = new InlineUIContainer(child);
-
-            Add(implicitRun);
-        }
-
-        public override void Add(Inline item)
-        {
-            if (!HasComplexContent)
-            {
-                if (!string.IsNullOrEmpty(_text))
-                {
-                    base.Add(new Run(_text));
-                }
-                             
-                _text = null;
-            }
-            
-            base.Add(item);
+            Add(new InlineUIContainer(child));
         }
 
         /// <summary>
@@ -152,7 +110,7 @@ namespace Avalonia.Controls.Documents
         /// </summary>
         protected void Invalidate()
         {
-            if(InlineHost != null)
+            if (InlineHost != null)
             {
                 InlineHost.Invalidate();
             }
@@ -162,7 +120,7 @@ namespace Avalonia.Controls.Documents
 
         private void OnParentChanged(ILogical? parent)
         {
-            foreach(var child in this)
+            foreach (var child in this)
             {
                 ((ISetLogicalParent)child).SetParent(parent);
             }
diff --git a/src/Avalonia.Controls/Documents/Span.cs b/src/Avalonia.Controls/Documents/Span.cs
index c7289dbc3f..3560996727 100644
--- a/src/Avalonia.Controls/Documents/Span.cs
+++ b/src/Avalonia.Controls/Documents/Span.cs
@@ -38,41 +38,17 @@ namespace Avalonia.Controls.Documents
 
         internal override void BuildTextRun(IList<TextRun> textRuns)
         {
-            if (Inlines.HasComplexContent)
+            foreach (var inline in Inlines)
             {
-                foreach (var inline in Inlines)
-                {
-                    inline.BuildTextRun(textRuns);
-                }
-            }
-            else
-            {
-                if (Inlines.Text is string text)
-                {
-                    var textRunProperties = CreateTextRunProperties();
-
-                    var textCharacters = new TextCharacters(text.AsMemory(), textRunProperties);
-
-                    textRuns.Add(textCharacters);
-                }
+                inline.BuildTextRun(textRuns);
             }
         }
 
         internal override void AppendText(StringBuilder stringBuilder)
         {
-            if (Inlines.HasComplexContent)
-            {
-                foreach (var inline in Inlines)
-                {
-                    inline.AppendText(stringBuilder);
-                }
-            }
-            else
+            foreach (var inline in Inlines)
             {
-                if (Inlines.Text is string text)
-                {
-                    stringBuilder.Append(text);
-                }
+                inline.AppendText(stringBuilder);
             }
         }
 
@@ -93,7 +69,7 @@ namespace Avalonia.Controls.Documents
         {
             base.OnInlineHostChanged(oldValue, newValue);
 
-            if(Inlines is not null)
+            if (Inlines is not null)
             {
                 Inlines.InlineHost = newValue;
             }
diff --git a/src/Avalonia.Controls/Documents/TextElement.cs b/src/Avalonia.Controls/Documents/TextElement.cs
index 5bac3642ed..2034f05fc0 100644
--- a/src/Avalonia.Controls/Documents/TextElement.cs
+++ b/src/Avalonia.Controls/Documents/TextElement.cs
@@ -18,7 +18,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="FontFamily"/> property.
         /// </summary>
         public static readonly AttachedProperty<FontFamily> FontFamilyProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, FontFamily>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, FontFamily>(
                 nameof(FontFamily),
                 defaultValue: FontFamily.Default,
                 inherits: true);
@@ -27,7 +27,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="FontSize"/> property.
         /// </summary>
         public static readonly AttachedProperty<double> FontSizeProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, double>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, double>(
                 nameof(FontSize),
                 defaultValue: 12,
                 inherits: true);
@@ -36,7 +36,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="FontStyle"/> property.
         /// </summary>
         public static readonly AttachedProperty<FontStyle> FontStyleProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, FontStyle>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, FontStyle>(
                 nameof(FontStyle),
                 inherits: true);
 
@@ -44,7 +44,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="FontWeight"/> property.
         /// </summary>
         public static readonly AttachedProperty<FontWeight> FontWeightProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, FontWeight>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, FontWeight>(
                 nameof(FontWeight),
                 inherits: true,
                 defaultValue: FontWeight.Normal);
@@ -53,7 +53,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="FontStretch"/> property.
         /// </summary>
         public static readonly AttachedProperty<FontStretch> FontStretchProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, FontStretch>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, FontStretch>(
                 nameof(FontStretch),
                 inherits: true,
                 defaultValue: FontStretch.Normal);
@@ -62,7 +62,7 @@ namespace Avalonia.Controls.Documents
         /// Defines the <see cref="Foreground"/> property.
         /// </summary>
         public static readonly AttachedProperty<IBrush?> ForegroundProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, IBrush?>(
+            AvaloniaProperty.RegisterAttached<TextElement, TextElement, IBrush?>(
                 nameof(Foreground),
                 Brushes.Black,
                 inherits: true);
diff --git a/src/Avalonia.Controls/RichTextBlock.cs b/src/Avalonia.Controls/RichTextBlock.cs
index 2b84113497..abd6e7e807 100644
--- a/src/Avalonia.Controls/RichTextBlock.cs
+++ b/src/Avalonia.Controls/RichTextBlock.cs
@@ -16,7 +16,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control that displays a block of formatted text.
     /// </summary>
-    public class RichTextBlock : TextBlock, IInlineHost
+    public class RichTextBlock : TextBlock, IInlineHost, IAddChild<string>, IAddChild<Inline>, IAddChild<IControl>
     {
         public static readonly StyledProperty<bool> IsTextSelectionEnabledProperty =
             AvaloniaProperty.Register<RichTextBlock, bool>(nameof(IsTextSelectionEnabled), false);
@@ -148,7 +148,6 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the inlines.
         /// </summary>
-        [Content]
         public InlineCollection Inlines
         {
             get => GetValue(InlinesProperty);
@@ -170,6 +169,8 @@ namespace Avalonia.Controls
             remove => RemoveHandler(CopyingToClipboardEvent, value);
         }
 
+        internal bool HasComplexContent => Inlines.Count > 0;
+
         /// <summary>
         /// Copies the current selection to the Clipboard.
         /// </summary>
@@ -251,25 +252,69 @@ namespace Avalonia.Controls
             SelectionEnd = SelectionStart;
         }
 
-        protected override string? GetText()
+        void IAddChild<string>.AddChild(string text)
         {
-            return _text ?? Inlines.Text;
+            AddText(text);
         }
 
-        protected override void SetText(string? text)
+        void IAddChild<Inline>.AddChild(Inline inline)
         {
-            var oldValue = _text ?? Inlines?.Text;
+            if (!HasComplexContent && !string.IsNullOrEmpty(_text))
+            {
+                Inlines.Add(_text);
+
+                _text = null;
+            }
+
+            Inlines.Add(inline);
+        }
 
-            if (Inlines is not null && Inlines.HasComplexContent)
+        void IAddChild<IControl>.AddChild(IControl child)
+        {
+            if (!HasComplexContent && !string.IsNullOrEmpty(_text))
             {
-                Inlines.Text = text;
+                Inlines.Add(_text);
 
                 _text = null;
             }
-            else
+
+            Inlines.Add(new InlineUIContainer(child));
+        }
+
+        protected void AddText(string? text)
+        {
+            if (string.IsNullOrEmpty(text))
+            {
+                return;
+            }
+
+            if (!HasComplexContent && string.IsNullOrEmpty(_text))
             {
                 _text = text;
             }
+            else
+            {
+                if (!string.IsNullOrEmpty(_text))
+                {
+                    Inlines.Add(_text);
+
+                    _text = null;
+                }
+
+                Inlines.Add(text);
+            }
+        }
+
+        protected override string? GetText()
+        {
+            return _text ?? Inlines.Text;
+        }
+
+        protected override void SetText(string? text)
+        {
+            var oldValue = _text ?? Inlines?.Text;
+      
+            AddText(text);        
 
             RaisePropertyChanged(TextProperty, oldValue, text);
         }
@@ -293,7 +338,7 @@ namespace Avalonia.Controls
 
             var inlines = Inlines;
 
-            if (inlines is not null && inlines.HasComplexContent)
+            if (HasComplexContent)
             {
                 var textRuns = new List<TextRun>();
 
diff --git a/src/Avalonia.Controls/TextBlock.cs b/src/Avalonia.Controls/TextBlock.cs
index 52261d1c76..8077c00faf 100644
--- a/src/Avalonia.Controls/TextBlock.cs
+++ b/src/Avalonia.Controls/TextBlock.cs
@@ -4,6 +4,7 @@ using Avalonia.Controls.Documents;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Media.TextFormatting;
+using Avalonia.Metadata;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls
@@ -11,7 +12,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control that displays a block of text.
     /// </summary>
-    public class TextBlock : Control
+    public class TextBlock : Control, IAddChild<string>
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
@@ -473,11 +474,6 @@ namespace Avalonia.Controls
             control.SetValue(MaxLinesProperty, maxLines);
         }
 
-        public void Add(string text)
-        {
-            _text = text;
-        }
-
         /// <summary>
         /// Renders the <see cref="TextBlock"/> to a drawing context.
         /// </summary>
@@ -512,6 +508,11 @@ namespace Avalonia.Controls
             TextLayout.Draw(context, new Point(padding.Left, top));
         }
 
+        void IAddChild<string>.AddChild(string text)
+        {
+            _text = text;
+        }
+
         protected virtual string? GetText()
         {
             return _text;
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguage.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguage.cs
index 54eac8f59e..c8fced515d 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguage.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguage.cs
@@ -70,8 +70,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 },
                 InnerServiceProviderFactoryMethod =
                     runtimeHelpers.FindMethod(m => m.Name == "CreateInnerServiceProviderV1"),
-                IAddChild = typeSystem.GetType("Avalonia.Markup.Xaml.IAddChild"),
-                IAddChildOfT = typeSystem.GetType("Avalonia.Markup.Xaml.IAddChild`1")
+                IAddChild = typeSystem.GetType("Avalonia.Metadata.IAddChild"),
+                IAddChildOfT = typeSystem.GetType("Avalonia.Metadata.IAddChild`1")
             };
             rv.CustomAttributeResolver = new AttributeResolver(typeSystem, rv);
 
diff --git a/src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj b/src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj
index c35cbc35c0..30d321426f 100644
--- a/src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj
+++ b/src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj
@@ -15,7 +15,6 @@
         <Compile Include="Converters\FontFamilyTypeConverter.cs" />
         <Compile Include="Converters\TimeSpanTypeConverter.cs" />
         <Compile Include="Extensions.cs" />
-        <Compile Include="IAddChild.cs" />
         <Compile Include="MarkupExtension.cs" />
         <Compile Include="MarkupExtensions\CompiledBindingExtension.cs" />
         <Compile Include="MarkupExtensions\CompiledBindings\ArrayElementPlugin.cs" />