def _percentage_contribution(box):
    """Return the percentage contribution of a cell, column or column group.

    http://dbaron.org/css/intrinsic/#pct-contrib

    """
    min_width = box.style['min_width'].value if box.style['min_width'
        ] != 'auto' and box.style['min_width'].unit == '%' else 0
    max_width = box.style['max_width'].value if box.style['max_width'
        ] != 'auto' and box.style['max_width'].unit == '%' else float('inf')
    width = box.style['width'].value if box.style['width'
        ] != 'auto' and box.style['width'].unit == '%' else 0
    return max(min_width, min(width, max_width))
