diff --git a/README.md b/README.md
index 4be2e28..2f08104 100644
--- a/README.md
+++ b/README.md
@@ -42,4 +42,5 @@ The problem in question will be demonstrated on a library application
 1. [Qt documentation for pyside2](https://doc.qt.io/qtforpython-5/PySide2/QtWidgets)
 1. [Qt documentation for PySide6](https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/)
 2. [Pythontutorial.net](https://www.pythontutorial.net/pyqt/pyqt-qtoolbar/)
-3. [Docs.sqlalchemy.org](https://docs.sqlalchemy.org/)
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+3. [Docs.sqlalchemy.org](https://docs.sqlalchemy.org/)
+4. [DTD tutorial](https://www.w3schools.com)
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