请问怎么在网页中实时显示textarea的字节？就是把下面的代码改成不用按BUTTON，直接动态显示字节数？我改成onchange=,但不知道为什么不行？
1<form>
2<textarea cols="50" name="SelfDescription" rows="10"></textarea><br/> <font color="#FF0000" face="Arial" size="1">Restricted to 50 minimum and 2000 maximum characters</font><br/>
3<input name="GetCount" onclick="form.CharacterCount.value=form.SelfDescription.value.length;" type="button" value="Count Characters ="/><input name="CharacterCount" readonly="" size="4" type="TEXT" value="0"/>
4</form>
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1<form>
2<textarea cols="50" name="SelfDescription" onpropertychange="form.CharacterCount.value=this.value.length" rows="10"></textarea><br/>
3<font color="#FF0000" face="Arial" size="1">Restricted to 50 minimum and 2000 maximum characters</font><br/>
4<input name="CharacterCount" readonly="" size="4" type="TEXT" value="0"/>
5</form>