Sorry, I forgot to say that we have to use width:100%.
Take a look at the code:
<f:facet name="footer">
<t:panelGrid columns="2" columnClasses="left_align, right_align ">
<h:commandButton value="LEFT_SIDE" action="" />
<h:commandButton value="RIGHT_SIDE" action="" />
</t:panelGrid>
</f:facet>
CSS
.left_align {
width: 100%;
text-align:left;
}
.right_align {
width: 100%;
text-align:right;
}
Greets.