<div class="firstclass<c:if test='true'> someclass</c:if>">
   <p>some other stuff...</p>
</div>

<c:if test='true'>
  <c:set value="someclass" var="cssClass"></c:set>
</c:if>
<div class="${cssClass}">
   <p>some other stuff...</p>
</div>

<div class="${booleanExpr ? 'cssClass' : 'otherCssClass'}">
</div>

<div id="loginById" style="height:<% out.print(instanceClass.booleanMethod()? "250px": "150px");%>">