If you have a set method of determining the popup "class", e,g. only features from a certain layer is returned, you can still do it in CSS:
.InfoWindowWidget .classA .title { display: none; }
To set the additional class of the popup:
var myIW = document.getElementsByClassName('InfoWindowWidget')[0];
myIW.className += " classA";
And to remove the additional class again:
var myIW = document.getElementsByClassName('InfoWindowWidget')[0];
myIW.className = "InfoWindowWidget";