You just list out what is needed in terms of your HTML output. I used this for my widgets:
array( 'br' => array( 'class' => array() ) , 'small' => array( 'class' => array() ) , 'h1' => array( 'class' => array(), 'style' => array() ) , 'h2' => array( 'class' => array(), 'style' => array() ) , 'h3' => array( 'class' => array(), 'style' => array() ) , 'h4' => array( 'class' => array(), 'style' => array() ) , 'h5' => array( 'class' => array() ) , 'h6' => array( 'class' => array() ) , 'li' => array( 'class' => array() ) , 'small' => array( 'class' => array() ) , 'ul' => array( 'class' => array() ) , 'a' => array( 'class' => array(), 'href' => array() ) , 'i' => array( 'class' => array(), 'style' => array(), 'id' => array() ) , 'div' => array( 'class' => array(), 'style' => array(), 'id' => array() ) , 'span' => array( 'class' => array(), 'style' => array(), 'id' => array() ) , 'img' => array( 'width' => array(), 'height' => array(), 'alt' => array(), 'src' => array(), 'class' => array(), 'style' => array(), 'id' => array() ) , 'href' => array( 'class' => array(), 'style' => array(), 'id' => array() ) )