Buttom loses style if aligned bottom

I simply want a button to be aligned bottom.
Apart from the fact that the following does not work, also the button loses its GWT style completely. What am I missing here?

<ui:style>
    .bottom {
        vertical-align: bottom;
    }
</ui:style>

<g:FlowPanel>
    <g:HTML>Placeholder<br/><br/></g:HTML>   
    <g:Button text="ads" styleName="{style.bottom}"/>
</g:FlowPanel>

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