IE9は、IE8までのIEで採用しているJavaScriptゲッターメソッド__defineGetter__およびセッターメソッド__defineSetter__をサポートせずに、ECMAScript 5仕様で定められているゲッターメソッドおよびセッターメソッドの表記のみをサポートすることになった。ブラウザごと ...

So far, we've learned how to build classes and give them instance methods. We also learned how to create instance variables and use setter and getter methods to set and retrieve their values, ...

There's a strange type issue in Text's style property due to how its getter and setter work: However, TS allows neither getter and setter declarations nor getters and setters with disagreeing types.