def border_image_outset(style, name, values):
    """Compute the ``border-image-outset`` property."""
    computed_values = [
        value if isinstance(value, (int, float)) else length(style, name, value)
        for value in values]
    if len(computed_values) == 1:
        computed_values *= 4
    elif len(computed_values) == 2:
        computed_values *= 2
    elif len(computed_values) == 3:
        computed_values.append(computed_values[1])
    return tuple(computed_values)
Compute the ``border-image-outset`` property.
weasyprint/css/computed_values.py