W3C Talk - slide "Example (1)"

<smil xmlns="http://www.w3.org/2000/SMIL20/CR/Language">
    <head>
        <customAttributes>
            <customTest id="fred" title="Fred sets the first image" 
               defaultState="false" override="visible" />
            <customTest id="barney" title="Barney sets the second image" 
                defaultState="true" override="visible" 
            />
        </customAttributes>
    </head>
    ...