Back in SharePoint 2010 the RSS Viewer web part (a.k.a RSS Aggregator web part, RSSAggregatorWebPart) supported the use of XSL templates which contained script tags. There were a few funny things you had to in order get it working but it did work. Bring on SharePoint Online and it is not possible to add a script tag to your XSL templates (I believe this ‘issue’ exists on SharePoint 2013 as well). More accurately, script tags can be added and will rendered to the page but the script will not be executed.
In this day and age the correct answer is very likely: “why are you using this web part?” or “XSL are you mad?”. Both very strong arguments, but regardless, for the record you can code (hack) your way around this issue.
Use the onerror handler of an image tag to instantiate your JS. Ensure that the img tag is rendered as the last element in your template if you plan on using the js to modify the DOM.
<img style='display:none' src='#' onerror='console.log("boo!")' />
If you are using SharePoint 2010 check this out: