//css:
p{
font-family:Caecilia LT Std 55 Roman;
font-size:35;
leading:10;
color:#252525;
}
.veryBig{
font-family:Caecilia LT Std 86 Heavy Italic;
font-size:77;
}
.bigOrange{
font-family:Caecilia LT Std 45 Light;
font-size:53;
color:#d55c19;
}
.bigBold{
font-family:Caecilia LT Std 85 Heavy;
font-size:53;
}
.normalThin{
font-family:Caecilia LT Std 46 Light Italic;
}
.bigThin{
font-family:Caecilia LT Std 45 Light;
font-size:58;
}

//ac3:
var url:URLRequest = new URLRequest("style.css");
var loader:URLLoader = new URLLoader();
loader.addEventListener(Event.COMPLETE, cssComplete);
loader.load(url);

function cssComplete(evt:Event){
	var css:StyleSheet = new StyleSheet();
	css.parseCSS(loader.data);
	wellcText.styleSheet = css;
	wellcText.htmlText = "<p><span class='veryBig'>Hello</span>, some text <span class='bigOrange'>some text</span>.<br>some text <span class='normalThin'>some text</span> my <span class='bigBold'>some text</span><br>some text <span class='bigBold'>some text</span><br>some text <span class='normalThin'>some text</span> some text <span class='bigThin'>some text</span>.</p>";
	wellcText.embedFonts=true;
	wellcText.selectable=false;
}

dont use spaces around the ":" in CSS its buggy :(

CSS for styling text in flash with nonstandard fonts

css, flash

ActionScript 3