In a:form,the

[chlol]

my page code :

<a:form id="login" focus="username" prependId="false">

 <rich:panel>
 <f:facet name="header">#{messages['label.login.title']}</f:facet>

 <p>#{messages['label.login.hint']}</p>

 <div class="dialog">
 <h:panelGrid columns="2" rowClasses="prop" columnClasses="name,value">
 <h:outputLabel for="username">#{messages['label.login.username']}</h:outputLabel>
 <h:inputText id="username"
 value="#{identity.username}"/>
 <h:outputLabel for="password">#{messages['label.login.password']}</h:outputLabel>
 <h:inputSecret id="password"
 value="#{identity.password}"/>
 <h:outputLabel for="rememberMe">#{messages['label.login.rememberMe']}</h:outputLabel>
 <h:selectBooleanCheckbox id="rememberMe"
 value="#{identity.rememberMe}"/>
 </h:panelGrid>
 </div>

 </rich:panel>

 <div class="actionButtons">
 <h:commandButton value="#{messages['button.login']}" action="#{identity.login}"/>
 </div>

 </a:form>

but when the page render,the focus is not in the "username"
why?