As discussed in CSS Best Practices and JavaScript Best Practices portlets require that all HTML element ids be namespaced to prevent duplicate ids on a page when a portlet is included multiple times on a page. Some JSP tag libraries automatically generate ids if they are not specified, so a developer should specify the ids. For example
<!-- Spring form tag will cause duplicate ids -->
<form:select path="title" items="${titles}" />
<!-- when rendered might look something like this -->
<select id="title" name="title">
<option value="Mr">Mr</option>
<option value="Mrs">Mrs</option>
<option value="Miss">Miss</option>
</select>
<!-- Instead specify a namespaced id -->
<c:set var="n"><portlet:namespace /></c:set>
<form:select id="${n}title" path="title" items="${titles}" />