The code you posted is AS 2 syntax. You have to convert it to AS 1 in order to use it.

Here's an AS 1 example that uses the stylesheet class.
Create a dynamic textfield with 'has html' set to yes.
Code:
var css_str = '.heading {font-size: 24px; font-weight: bold; }';

var my_styleSheet = new TextField.StyleSheet();
my_styleSheet.parseCSS(css_str);

txt1.styleSheet = my_styleSheet;
txt1.htmlText = "<p class='heading'>My formatted text</p>";
Wilbert