diff --git a/lib/models/documents/attribute.dart b/lib/models/documents/attribute.dart
index 4fae59b0..79ad5174 100644
--- a/lib/models/documents/attribute.dart
+++ b/lib/models/documents/attribute.dart
@@ -66,23 +66,23 @@ class Attribute<T> {
 }
 
 class BoldAttribute extends Attribute<bool> {
-  BoldAttribute() : super('bold', AttributeScope.INLINE, null);
+  BoldAttribute() : super('bold', AttributeScope.INLINE, true);
 }
 
 class ItalicAttribute extends Attribute<bool> {
-  ItalicAttribute() : super('italic', AttributeScope.INLINE, null);
+  ItalicAttribute() : super('italic', AttributeScope.INLINE, true);
 }
 
 class UnderlineAttribute extends Attribute<bool> {
-  UnderlineAttribute() : super('underline', AttributeScope.INLINE, null);
+  UnderlineAttribute() : super('underline', AttributeScope.INLINE, true);
 }
 
 class StrikeThroughAttribute extends Attribute<bool> {
-  StrikeThroughAttribute() : super('strike', AttributeScope.INLINE, null);
+  StrikeThroughAttribute() : super('strike', AttributeScope.INLINE, true);
 }
 
 class LinkAttribute extends Attribute<String> {
-  LinkAttribute() : super('link', AttributeScope.INLINE, null);
+  LinkAttribute() : super('link', AttributeScope.INLINE, '');
 }
 
 class HeaderAttribute extends Attribute<int> {
@@ -109,9 +109,9 @@ class ListAttribute extends Attribute<String> {
 }
 
 class CodeBlockAttribute extends Attribute<bool> {
-  CodeBlockAttribute() : super('code-block', AttributeScope.BLOCK, null);
+  CodeBlockAttribute() : super('code-block', AttributeScope.BLOCK, true);
 }
 
 class BlockQuoteAttribute extends Attribute<bool> {
-  BlockQuoteAttribute() : super('blockquote', AttributeScope.BLOCK, null);
+  BlockQuoteAttribute() : super('blockquote', AttributeScope.BLOCK, true);
 }
diff --git a/lib/models/documents/leaf.dart b/lib/models/documents/leaf.dart
new file mode 100644
index 00000000..a785e6b2
--- /dev/null
+++ b/lib/models/documents/leaf.dart
@@ -0,0 +1,6 @@
+import 'node.dart';
+
+/* A leaf node in document tree */
+abstract class Leaf extends Node implements StyledNode {
+
+}
diff --git a/lib/models/documents/node.dart b/lib/models/documents/node.dart
new file mode 100644
index 00000000..a6cb7e39
--- /dev/null
+++ b/lib/models/documents/node.dart
@@ -0,0 +1,41 @@
+import 'dart:collection';
+
+import 'package:flutter_quill/models/documents/style.dart';
+
+import 'attribute.dart';
+
+/* node in a document tree */
+abstract class Node extends LinkedListEntry<Node> {}
+
+abstract class StyledNode implements Node {
+  Style get style => _style;
+  Style _style = Style();
+
+  void applyAttribute(Attribute attribute) {
+    _style = _style.merge(attribute);
+  }
+
+  void applyStyle(Style value) {
+    if (value == null) {
+      throw ArgumentError('null value');
+    }
+    _style = _style.mergeAll(value);
+  }
+
+  void clearStyle() {
+    _style = Style();
+  }
+}
+
+abstract class Container<T extends Node> extends Node {
+  Container _parent;
+
+  Container get parent => _parent;
+
+  bool get isFirst => list.first == this;
+
+  bool get isLast => list.last == this;
+}
+
+/* Root node of document tree */
+class Root extends Container<Container<Node>> {}
diff --git a/lib/models/documents/style.dart b/lib/models/documents/style.dart
index 6843ed50..d5e6944d 100644
--- a/lib/models/documents/style.dart
+++ b/lib/models/documents/style.dart
@@ -33,4 +33,22 @@ class Style {
   Attribute get single => _attributes.values.single;
 
   bool containsKey(String key) => _attributes.containsKey(key);
+
+  Style merge(Attribute attribute) {
+    Map<String, Attribute> merged = Map<String, Attribute>.from(_attributes);
+    if (attribute.value == null) {
+      merged.remove(attribute.key);
+    } else {
+      merged[attribute.key] = attribute;
+    }
+    return Style.attr(merged);
+  }
+
+  Style mergeAll(Style other) {
+    Style result = Style.attr(_attributes);
+    for (Attribute attribute in other.values) {
+      result = result.merge(attribute);
+    }
+    return result;
+  }
 }