MarkupRenderer is an object which will perform rendering of a page (or portion of a page). This interface exists to be filtered via MarkupRendererFilter.The MarkupRenderer service takes an ordered configuration of MarkupRendererFilters, which are used for ordinary page rendering (as opposed to partial page rendering for Ajax requests).
In practice, if you want to use your portlet more than once in your portal page, don't fix the client id in your template.
Instead, let's the component create a unique client ID with the namespace provided by t:id parameter.
and get the clientID form the component
If you want more details about What's the difference between id and t:id? see http://tapestry.apache.org/templating-and-markup-faq.html